Declan Akaba

Declan Akaba, known as Reiji Akaba ( Akaba Reiji) in the Japanese version, is one of the main characters and the tritagonist in the Yu-Gi-Oh! ARC-V anime. He is the second CEO of the huge Leo Corporation. In promotional materials for ARC-V, it was stated that Declan is the youngest Duelist in history to be certified to Duel in the professional circuit, having done so at age 15. However, while his skills are incredible and even greater than the World Champion at the beginning of the series by an immense margin, it was never stated in the actual show that Declan is a pro.

Declan functions as Yuya Sakaki's primary rival, despite their current alliance, at first due to his capability to Pendulum Summon, and later due to his questionable methods for resolving the Interdimensional Domination campaign. While Declan notes that Yuya has surpassed LID's top team, he still does not consider Yuya to be on equal standing with him. Despite his early introduction into the series, Declan's name was not mentioned on-screen until episode 8. He is the founder of the Lancers, an organization dedicated to fighting against invaders from other dimensions.

Appearance
Declan is a thin young man with pale skin and grey hair, the front layer being a lighter grey than the back. His hair is arranged in a simple style, flicking upwards at the ends. Declan's eyes are purple, and he wears red-framed spectacles. His ears are both pierced with black stud earrings. Despite his status as the CEO of the Leo Corporation, Declan dresses rather casually, wearing a dark-blue sweater, white pants that end just above his ankles, and white shoes (apparently without any socks beneath them), along with a long red scarf wrapped around his shoulders with the ends trailing to the small of his back. When he was monitoring the You Show Duel School in person, he concealed his identity beneath a large hooded sweatshirt.

He dressed much the same as a youth three years previously, but did not wear a scarf nor have his ears pierced, and he wore a darker blue sweater with a zig-zag pattern on the chest.

Personality
In general, Declan has a grim and serious demeanor, but he takes an interest in Yuya Sakaki. He is utterly polite without fail, save rare moments of a loss in composure. Unlike other rivals from the previous series Declan is calm and collected, and is shown to be extremely level-headed and strong-willed. Despite his advanced skills, he is not overly arrogant, merely truthful. He is quite respectful to Yuya's father Yusho Sakaki due to his history with Leo Akaba, fiercely defending him when he was insulted in a rare moment of emotion. While he does not consider empathy as beneficial on the battlefield, he does seem to enjoy himself while Dueling, and even requested Yuya to demonstrate Yusho's Dueltaining. He was shown to smile when Yuya exerted his full strength. He later admits that while Yuya's feelings gave birth to a great power, this is not always the case and emotions can still cost one on the battlefield.

Despite his declared rivalry with Yuya Sakaki, Declan does not show any open dislike for him, and in fact defended him from false accusations of attacking Leo Corporation employees and later remained confident that Yuya wouldn't be foolish enough to cooperate with Jean-Michel Roget; this demonstrates that he may hold a degree of respect for Yuya. Declan has also mentioned that he does not care very much for his father, Leo Akaba, and he demonstrates an intense dislike of his father's ambitions, given the suffering that they have caused. Declan is also pragmatic without regard for the actions of potential allies, offering an alliance with Shay and Celina despite their attacks on his students, and to Sora Perse after Sora renounced Duel Academy, but doesn't extend similar offers to people with ulterior motives like Jean-Michel Roget.

He is shown to be highly analytical and a skilled strategist, and claims he only plays games that he knows that he can win. He used the Arc League Championship as a test to recruit Lancers, and came up with a plan to fend off the Fusion Dimension invasion within only a day. He can show a less moral side when executing his plans, as he had no problem allowing many Duelists to be sealed in cards during the Battle Royal and only cared about the ones that did survive. During his second Duel with Yuya he showed a cruel indifference and revealed that he considers feelings to be a weakness and a tool to manipulate others, as seen when he used Yuya's feelings towards Zuzu to push him to Duel more efficiently by lying about Zuzu's fate after her disappearance.

Despite his cold nature, he cares closely about his adopted younger sister Riley and gets along with his mother. According to Moon Shadow, Declan does care about others' lives and won't abandon his allies, even if his actions seem to suggest otherwise; he sent Moon underground precisely for this purpose. In the Friendship Cup arcs, Declan tends to act more reserved and quiet and takes action only when it is necessary. However he later criticized the High Council for their incompetence in keeping New Domino City under control and Roget for his selfish ambitions.

In his youth, Declan was more reckless, crossing dimensions (though unaware that he had done so at the time) to find his father after he had disappeared and exposing his hiding place to save Celina. He also spoke far more brashly (in Japanese, he referred to himself as "boku" in contrast with his current self who refers to himself as "watashi", which is the polite form), demanding answers from his father. Declan made the switch in pronouns after making the decision to take over as President of the Leo Corporation.

Etymology
His character's last name, Akaba, means "Red Horse". Declan's surname is similar to that of Seto Kaiba from Yu-Gi-Oh!, whose name also has also contains the character 馬 (Uma, Horse), serving as another similarity between the two characters. His given Japanese name of Reiji means "Zero Child"; he shares the chaarcter of "Zero" with his father, adoptive sister and Ray Akaba. With a different kanji(怜侍), it can also mean "cleverness".

His dub name, Declan, is likely come from René Descartes, a French philosopher, mathematician, and scientist. A theorem named after Descartes (Descartes' theorem) also involves connection with circles, connecting him with his Japanese name's meaning.

Abilities
During his first Duel with Yuya, Declan demonstrated swift athleticism when necessary. He also possesses good reflexes and strength as he managed to intercept a punch from the athletic Yuya with ease.

History
Three years before the Arc League Championship, Declan was worried about his mother's change after his father's disappearance, so he decided to go find Leo in order to confront him about her sudden change. He used a strange teleporting decive and appeared inside Duel Academy. He wandered the halls until he noticed Celina being chased by faculty members. When they cornered her, he watched from atop a wall behind her as she Dueled against them, and was surprised to see that their holograms had mass without an Action Field. He jumped in, Summoning "D/D Cerberus" to defend her. He then offered her his hand, which she took, and pulled her up over the wall. They fled to the harbor, where they were caught by Leo Akaba and several Obelisk Force members, to Declan's surprise.

Celina mentioned that Leo wanted to destroy the Xyz and Synchro Dimensions, mentioning that the attack on the Xyz Dimension was about to start, prompting a confused Declan to protest that Leo had not mentioned anything to him or his mother. The Obelisk Force squad then took Celina away, and then one of them placed a miniature teleportation device on Declan's wrist, which sent him back to the Standard Dimension and then self-destructed.

These events motivated Declan to stop his father's ambitions. He became the President of the Leo Corporation and managed to achieve entry into the Professional Circuit by the age of fifteen. One night at Leo Corporation, Declan asked Yusho to lead the Lancers and help him stop Leo's ambition after investigating the research his father left behind. However, Yusho never returned. At some point, an orphaned boy from a war-torn country was brought in and his mother explained that the boy had lost his sense of self from what he had experienced. This made him an ideal candidate to become a Duel Soldier to fight against Duel Academy. Declan, however, was quite disturbed by this. The boy was adopted into their family, and Declan developed a caring relationship with his adopted brother.

Declan, knowing what Riley's life had been like, gave him "C/C Critical Eye", telling Riley that his life had no light in it, so he must capture the light of his opponents. He then taught Riley, who quickly became a strong Duelist.

Pre-Arc League Championship
After Yuya Sakaki defeated The Sledgehammer using the Pendulum Summon mechanic, Declan was briefed on the Summon and on Yuya himself by Claude. Intrigued by the Pendulum Summon, Declan asked Sylvio Sawatari to learn what he could about Pendulum Summoning, no matter how small the detail and to acquire them, if possible. Sylvio successfully acquired the cards, but wanted to use them to Duel Yuya. As Claude protested from where he was watching the Duel, Declan arrived and assured him that it was all right. Declan advised Sylvio on the use of Yuya's Pendulum Monsters from behind the scenes throughout the Duel. After Sylvio's loss, Claude stated that they would need to use other means to seize the cards immediately, but Declan again reassured him that it was all right for now. He looked at a video feed of Yuya, thanking him for putting on a show and murmured Yuya's name.

A week after the first Pendulum Summon, his mother Henrietta arrived back from a business trip. Declan wasted little time in informing her about Pendulum Summoning and Yuya Sakaki, and stated that they'd almost achieved a complete understanding of the Summon. He also informed her of the supposed attack on Sylvio at Yuya's hands. Declan admitted to being worried that, despite the feat he had performed, Yuya only had a 50% win-loss-ratio, and he was the son of Yusho Sakaki, who given his disappearance after departing three years previously had possibly allied with enemies of Leo Institute of Dueling. Later, he observed the Duels between the You Show Duel School and LID, remaining hidden and concealing his identity beneath a hooded sweatshirt. After Yuya had defeated Dipper, Zuzu had been defeated by Julia, and Gong had tied with Kit, Henrietta angrily proposed a tiebreaker between the students that had won: Yuya and Julia. Before the situation could progress any further, Declan revealed himself, and he stated he would put an end to it.

None of the students present recognized Declan, his mother commenting that they didn't know how strong he was. Declan requested that she and their students stay quiet and watch, and that they not cheer any further. Declan allowed Yuya to choose whichever Action Field he wanted, so Skip Boyle (the only You Show member aware of Declan's identity) picked Yuya's favorite Field, "Acrobatic Circus." In exchange, Yuya let Declan go first. Declan proceeded to use three "Dark Contract" Continuous Spell Cards to Fusion Summon "D/D/D Flame King Genghis" on his turn (this is his first use of a Fusion Monster to be known, and there were no record of him using them during any of his past Duels), then Set the rest of his hand, but leaving him apparently guaranteed to take 3000 damage on his next turn. When Yuya attempted to retaliate, Declan activated another "Dark Contract" card that would cause him to take another 1000 damage during the next turn. Yuya was able to counter that attack as well, but Declan was well-prepared, using an Action Card to protect "Genghis." Yuya was given the option to end his turn and theoretically win, but hesitated. Declan observed that Yuya's kindness was causing him to hesitate, though he called it useless in battle. Yuya Set a card, then Declan negated and destroyed his "Dark Contract" cards with "Contract Laundering", replenishing his hand in addition. Yuya admitted that he wouldn't have been happy if he'd won from that, as he wouldn't have gotten a chance to show off his father's Dueltaining style. Declan asked if he was referring to Yusho Sakaki, and Yuya was surprised that Declan seemed to know his father. Kit mocked Yusho from the sidelines, prompting Declan to roar at him to be silent. He apologized for his outburst, explaining that he respected Yusho from the bottom of his heart. Declan then proceeded to Synchro Summon "D/D/D Gust King Alexander" and Xyz Summon "D/D/D Wave King Caesar" on his next turn, much to the shock of the onlookers.

Declan proceeded to destroy all of Yuya's monsters, despite Yuya's use of "Hippo Carnival". Yuya proceeded to Pendulum Summon on his next turn, using his "Odd-Eyes Pendulum Dragon" in combination with "Performapal Fire Mufflerlion" and an Action Card to destroy all three of Declan's monsters. Declan used the effect of "Caesar" to revive them, though at the cost of taking damage again on his next turn, and then he Tributed them with "D/D/D Human Resources" - bypassing the damage he would have taken - to add his "D/D Savant Galilei" and "D/D Savant Kepler" to his hand - Pendulum Monster prototypes that he proceeded to activate on his next turn. He congratulated Yuya after Yuya claimed that he looked forward to seeing whatever surprises Declan had in store, Declan asking Yuya to confirm whether Pendulum Summoning was unique with his own eyes. Declan then Pendulum Summoned three copies of "D/D/D Doom King Armageddon", destroying all of Yuya's monsters again and reducing him to 350 Life Points, also discovering, to his surprise, that destroyed Pendulum Monsters were sent to the Extra Deck instead of the Graveyard. Yuya demanded to know where Declan had acquired Pendulum Summoning, but Declan refused to answer. Yuya Pendulum Summoned his destroyed monsters back, along with a new one, "Performapal Partnaga", and attempted to destroy all three "King Armageddons", but the destruction of one simply activated the effects of the others, which gained the ATK of the destroyed monsters. Before Declan could counterattack, his Pendulum Monsters' effects appeared and reduced their Pendulum Scales, destroying his remaining "King Armageddons." He burst out laughing in surprise, realizing that Pendulum Summoning was not complete and that he had seen the possibility for further evolution in Pendulum Summoning. His mother then called out his name after hearing about the disappearance of Professor Marco, and Claude briefed Declan on what had happened. Akaba told Yuya that he would settle the schools' matters some other time. Declan gave Yuya the win, and when asked, told Yuya his name before departing. Arriving back at LID, he learned that Marco had been involved in a Duel with an Xyz user. As they'd been with Yuya at the time, and Yuya had not demonstrated the capability, this ruled him out as the culprit. Declan ordered his men to search for Marco after being shown Marco's cards and Duel Disk.

Following this, Declan anonymously requested that Yuya be allowed into the Junior Arc League Championship. Yuya, however, declined, and won his first Duel of the required four to gain entry normally. Claude later informed Declan of this, and Declan smiled, commenting that it seemed he'd done something unnecessary, that he should have expected it of the originator of Pendulum Summoning, and he asked if Yuya had Pendulum Summoned and if there was any change. Claude replied that there wasn't and admitted that he was bothered by elevated levels of Fusion Summoning that were occurring. The readings were stronger than normal, and Declan compared it to the abnormally strong Xyz Summonings that had occurred recently. He ordered it to be monitored. His mother later approached him with two cards that had been sent anonymously to LID that morning depicting Marco and Herk, one of LID's aces. Declan mused that the previous day, another strong Xyz Summon had been detected, and deduced that the two had been sealed inside the cards, though this was the first time he'd witnessed such a phenomenon. Though Marco and Herk were alive, they couldn't be saved with LID's current resources. He commented that this did raise one question - there had been there Xyz-related attacks, yet the first victim - Sylvio - hadn't been sealed in a card, suggesting two perpetrators. Before they could continue their talk, Curio Sawatari barged into the office furiously, asking why they were loafing around. Despite Mr. Sawatari not having an appointment, Declan acknowledged that the rumors of the third attack were true. Curio angrily asked why they weren't taking action to catch the perpetrator, pointing out that his son had testified that Yuya Sakaki had attacked him. Declan asked if Sylvio was still saying that, and Curio admitted that he hadn't seen Sylvio since he'd left the hospital, but hadn't heard Sylvio say differently. He asked why they were letting Yuya run free, despite Henrietta wanting to handle it herself, and wanted to inform the police. Rising from his chair, Declan warned him that it was under investigation. Mr. Sawatari insisted that Yuya was the perpetrator, but Declan forcefully told Mr. Sawatari that Yuya was no criminal. He vowed to catch the real criminal, until then, he'd like Mr. Sawatari to watch quietly. After Curio Sawatari had left the office in a huff, Declan ordered Claude to raise the level of alert in the inner city and analyze the cards of Marco and Herk to save all of the city's Duelists. He later learned that the strong Fusion Summoning readings were continuing, and mused that the person using them must be winning as well.

Later, Declan practiced Pendulum Summoning again, this time with the effects of his cards updated, and he successfully Summoned all three "King Armageddons." Imagining himself facing Yuya and "Odd-Eyes Pendulum Dragon", Declan reached for his next card, but before he could continue his move, he was advised to stop, as they had cleared the day's benchmarks and the experiment was a success. Claude congratulated him, stating that now they were one step closer to creating even more of the Leo Corporation's own Pendulum Cards. Despite this, Declan considered his own inferior in terms of activation energy output to Yuya's Pendulum Monsters, believing them to be incomplete and unusable in the tournament. He gave Claude until the tournament to fix the problem, as they could not underestimate their competition, no matter where it came from.

When Claude was complaining about using students to draw out the Duelist that had been attacking their students, Declan arrived and ordered them to continue monitoring them, confident that whoever it was would show themselves. He witnessed Shay admit to attacking the LID students and the start of his Battle Royal against Julia, Kit and Dipper. When Shay defeated all three of the students, Leo Corporation staff arrived and Shay asked them to see their boss. Declan replied that he was right there if Shay wanted him, and his employees parted as Declan walked towards the Xyz-user. Declan explained to Shay that he was the current Leo Corporation President, and Shay immediately challenged him to a Duel. Declan requested to know why Shay was attacking them. Though Shay refused to answer, Declan had deduced from his actions and Julia's prior testimony that Shay was trying to rescue his comrades by using Declan as a hostage. Shay confirmed that his attacks had been to lure Declan out to persuade Leo Akaba to release his comrades. Declan claimed that he doubted his father valued him that much, but agreed to Duel Shay under conditions: Shay must enter the Junior Arc League Championship and fight certain Duelists for him, informing him in a roundabout manner of Sora Perse, a Fusion user, and Yuya, the user of the Pendulum Summoning that Shay would likely be unfamiliar with. Declan explained that he too wanted to fight against his father and clarified that if Shay won, he could do whatever he wanted. He departed with his men afterwards. Claude later confirmed that Yuya had successfully entered into the competition and that Shay had been wiped from Julia, Dipper and Kit's memories, and entered into the Junior Arc League tournament. Declan mused that all that was left was to see how much power Shay would display, and he looked through his Lancers Selection document, muttering that the same question applied to the rest of the Duelists in the folder - which of them would become the Lancers to fight for their world?

Arc League Championship
At the beginning of the tournament, Declan made one of his first public appearances in a while, much to the delight of his fans. He, his mother, and Riley watched the opening proceedings, observing the entry of both Yuya and Shay, and after Yuya's brief mess-up and subsequent speech regarding Yusho Sakaki, Declan led the applause. He watched Riley's first Duel against Allie, and congratulated his little brother after he won.

Before Sylvio's Duel with Yuya, Declan gave him his new "Yosenju" cards, including some Pendulum Monsters, and taught him an anti-Pendulum combination that would return Pendulum Cards to the Deck. He monitored the Summoning, which was successful, as was Sylvio's use of the technique. He was pleasantly surprised when Yuya Fusion Summoned using a Pendulum Monster as the material, pleased that Yuya had already evolved so far. After Sylvio's loss, he ordered Claude to retrieve his Pendulum Cards and analyze them for mass-production.

Before Shay's Duel with Sora, Declan arranged for the Action Field "Neo Heartland City" to be used to light Shay's fighting spirit. Declan also had a Pendulum Card made for Shay, who rejected it. While this annoyed Claude, Declan was fine with the rejection of his gift, as he was more interested in Shay's abilities rather than the potential of Pendulum Summoning. He was quite surprised when Shay performed a Rank-Up Xyz Evolution during the Duel. When Shay called out "Raidraptor - Revolution Falcon," he told Riley to watch their future at stake. After Shay defeated Sora, he told him not to take further action in the eyes of the public, as Sora would be useful for experiments to understand Duel Academy.

Sora escaped during the night, and Declan coordinated the search for him, finding him in Central Park with a man who resembled Yuya Sakaki. Shay explained that this was his comrade, Yuto, the second Xyz user that Declan had suspected of attacking LID. He refused to let Shay assist Yuto, and they simply watched the Duel between Sora and Yuto, and the subsequent arrival and joining of Yuya. After Sora revealed himself as an agent of Duel Academy, his Duel Disk transported him away. Declan watched as Yuto explained to Yuya about the Four Dimensions, growing angrier as he heard about the attacks on Yuto's homeland, and explaining that it was due to his father's ambition that the tragedy had occurred. Before he could learn more, the security camera they were accessing was damaged. Declan still refused to let Shay go, warning him that he would protect his city in his own way. When they detected a Synchro Summoning reaction, Shay explained that Yuto must be Dueling the Pawn of Fusion. Claude later found Yuto's Duel Disk, and Declan ordered it brought to him as he looked out over Paradise City. Claude did as ordered, and Declan proceeded to scan it for information.

Declan witnessed Yuya's next Duel with Iggy Arlo, suspecting that Yuya had possession of Yuto's "Dark Rebellion Xyz Dragon"; his suspicions were confirmed when Yuya Xyz Summoned it. When a stunned Shay turned to go and find Yuya, Declan reminded him of the tournament's purpose before he left. Declan then heard Nico Smiley announce that Dipper had lost his Duel to Reed Pepper by default due to absence. This confused Declan, until he was informed that a Fusion Summoning reaction had been detected. He was shown security camera footage that caught Dipper being sealed inside a card by Celina, whose face he recognized. He then learned that Celina had watched Kev Ravenwood's Duel and realized that Kev would be Celina's next target, so he devised a plot to use Kev as bait to lure Celina, which worked perfectly. Declan immediately told Kev to leave, and as the latter turned to do so, Celina chased him, until Declan stopped her by grabbing her arm. This prompted Celina to threaten to take Declan out first, until Barrett, Celina's cohort, challenged Declan instead. Declan prepared his Duel Disk, ordering Claude through a communication link to seal off the area. Declan inferred from Barrett's use of Medal Counters and willingness to take risks that Barrett had extensive combat experience, which Barrett confirmed. Though Declan eventually took damage from his own "Dark Contract with the Swamp King" card, earning a respectful comment from Barrett, this damage ultimately sealed Declan's victory, as he proceeded to dispatch Barrett using "D/D/D Marksman King Tell" and "D/D/D Oracle King d'Arc". Celina turned to leave, until Declan told her to wait.

He commented that she wasn't Zuzu Boyle, and Celina asked who Zuzu was. Declan explained that Zuzu was someone from the Standard Dimension that resembled Celina and introduced himself, asking her if she remembered him and if she had been sent by Leo Akaba as a vanguard for an invasion. Celina denied working with Leo, having come to Standard of her own free will, and she admitted that she remembered Declan too, as she'd been trapped in that castle since Declan's appearance in her world. Declan explained that he was opposing Leo and offered an alliance, but Celina refused, explaining that she was after the Xyz users in Standard under the belief that defeating them would make Leo Akaba acknowledge her abilities. She gave Declan the card containing Dipper's soul, and Declan informed her that neither Dipper nor Kev were her target, though Akaba claimed to not know the Duelist from the Xyz Dimension who had defeated Sora. Declan warned Celina that he would be considering her an enemy even if she wasn't working with Leo and would take appropriate action. Barrett, overhearing this, activated the forced return function of his Duel Disk, returning to the Fusion Dimension, but alerting them to Celina's location. Celina explained that there would be people from Duel Academy coming after her, which would cause more than just a disturbance, but claimed before departing that once she'd defeated them and the Duelist from the Xyz Dimension, Leo would have no choice but to acknowledge her abilities.

To prepare for the impending invasion, Declan organized the third round of the Championship as a Battle Royal between the 16 remaining competitors using the "Quartet of Quandry" Action Field. This was because he was worried that simply warning the citizens would cause a panic. He intended for the Top 8 Duelists from the Senior Championship to intercept the invaders, and for the Junior Youth Duelists to take them out. He instructed Shay to monitor the Junior Youth Duelists and seek out those who could join the Lancers in case of an emergency, to which Shay argued that he'd have to abandon those orders if the Senior Duelists all got defeated. Declan later returned to his control room to monitor the proceedings. He cut the video feed from the Ancient Ruins Area of the Action Field to the stadium when Obelisk Force appeared, followed by the Volcano Area when Celina showed up, then ordered the Senior Duelists to move out. He took note of a powerful Synchro Summoning reading in the iceberg area, theorizing that it was the Pawn of Fusion that Shay had spoken of. When the Obelisk Force appeared before Celina, he ordered the Senior Team to be sent to the Volcano Area. The Obelisk Force whittled down the Senior Team until only Kev remained, though he was saved by a Duelist from the Synchro Dimension who wiped out the entire Obelisk Force. Declan realized from this that the Synchro Dimension wasn't their enemy, and that LID wasn't skilled enough to resist Duel Academy. He theorized that the Duelists from Yuya's generation might be able to become stronger due to entering the new stage of Dueling that Pendulum Summoning had brought about. Sylvio then barged into the control room, demanding to know what was going on. Declan decided to send Sylvio into the Battle Royal to provide assistance, filling him in on the situation (though apparently, not of the intrusion penalty for the Battle Royal.) He also witnessed the transportation of Zuzu and the Duelist from the Synchro Dimension. After the timer for the Battle Royal expired, Declan met the survivors - Yuya, Sylvio, Gong Strong, Celina, Shay, Dennis McField and Moon Shadow. He informed them that he had seen all of their Duels.

Friendship Cup
Declan revealed that he had changed the third round of the tournament to a city-wide Battle Royal to drive the Obelisk Force away with their abilities. Since they had managed to win and survive, he stated that they should be proud of their triumph over such a strong enemy, but Yuya instead blamed him for Zuzu (Yuya did not know yet), Reed, Trout, and everyone else who had been sealed into cards. Yuya then tried to punch Declan, but Declan easily caught and repelled the attack, saying that Yuya should express his anger with a Duel, not with his fists, and that if Yuya won, he would repent his actions until Yuya was satisfied.

He explained that he was determined to rally the Lancers to protect their world from the same tragedy that befell the Xyz Dimension, while activating "Neo Heartland City". Declan requested that Shay explain what had happened to the Xyz Dimension, but Shay was unable to do so out of shame. Declan began to explain it himself, but was interrupted by Yuya (and Yuto's spirit within Yuya) angrily commanding him to stop talking and start Dueling. During the first turn, Declan managed to Summon the same Fusion, Synchro and Xyz Monsters he Summoned during his last Duel against Yuya. After Yuya Summoned "Rune-Eyes Pendulum Dragon" and "Beast-Eyes Pendulum Dragon", Declan hypothesized that Yuya's ability to Fusion Summon was due to Sora's strength affecting Zuzu, and in turn Yuya, though he reassured Yuya that it was merely a hypothesis. After managing to survive Yuya's attack, Declan told Yuya that he understood how Yuya felt about Zuzu, concluding that it was those feelings that enabled Yuya to Pendulum-Fusion Summon, but growing through feelings alone wouldn't be enough to defeat Duel Academy. When Yuya protested that he didn't want to become a Lancer, Declan asked if that meant he didn't want to save Zuzu, which prompted Yuya to believe that Zuzu hadn't been sealed into a card. Declan then told Yuya to defeat him if he wanted to save Zuzu and defeat Duel Academy.

Declan Pendulum Summoned on his next turn, bringing out "Oblivion King Abyss Ragnarok", using it to revive "Caesar" and banish "Rune-Eyes". He then fused his two "D/D/D" monsters into "D/D/D Wave Oblivion King Caesar Ragnarok" and destroyed "Beast-Eyes". He urged Yuya to show him his full strength, as he hadn't gone all out either. Yuya proceeded to Pendulum-Xyz Summon on his next turn, bringing out "Dark Rebellion Xyz Dragon" and nearly defeated Declan, but Declan was able to survive with the effect of "Caesar Ragnarok." He brought out "Armageddon" on his next turn, along with "Abyss Ragnarok" from the Extra Deck, and Pendulum-Xyz Summoned himself to call out "D/D/D Duo-Dawn King Kali Yuga" and destroyed "Dark Rebellion". He wondered what bonds Yuya's Pendulum-Xyz Summon had been born from and learned that Yuya was fighting not only to save Zuzu but also to keep a promise to Yuto. Yuya revived "Dark Rebellion" and used his "Xiangke Magician" and "Xiangsheng Magician" to overlay "Dark Rebellion" and "Odd-Eyes" to Xyz Summon "Odd-Eyes Rebellion Dragon". Upon seeing this, Declan smiled, declaring that this was what he wanted to see. Yuya launched a winning attack, thinking that he'd sealed Declan's Set cards, but the immunity of "Kali Yuga" allowed Declan to destroy and re-Set his cards, then use them to have "Kali Yuga" defeat "Rebellion Dragon", winning him the Duel. He reminded Yuya that he was naive, but claimed that he still had hopes for him, and all of the Lancers, who he'd chosen to cross dimensions with him. Walking away, he asked Claude if he'd collected the data from the Duel, and Claude replied that he had.

After Henrietta revealed the existence of the different dimensions, Declan outlined the Lancers to the public, also effectively turning it into a propaganda advertisement for LID, which would train the next generations of Lancers. He provided new clothes for Celina and she stayed under his protection. The next day, Declan gathered the Lancers at Leo Corporation, informing them that they would be going to the Synchro Dimension to recruit allies in their fight against the Academy and also explained that Riley would be going with them. He explained that he'd used the technology in Yuto's Duel Disk to upgrade those of the Lancers to travel between dimensions and project an Action Field, and Celina forced him to admit that he'd seen Zuzu be transported to the Synchro Dimension. The Lancers then departed.

When the Lancers arrived they were split into several groups. Declan's whereabouts were unknown to most of the Lancers and several of them attempted to find him for days with no success. When Moon Shadow rescued Riley from Security Celina suggested that Declan instructed him to do that. After the Lancers were taken to the High Council and Yuya claimed they did not want to cause trouble, Declan appeared with Moon Shadow and Riley. He revealed to Jean-Michel Roget that they came to the Synchro Dimension in order to form an alliance. To help reinforce the Lancers' trust with the Synchro Dimension, Declan suggested they take part in the Friendship Cup, which Jean agreed to.

Declan, with Moon Shadow, Riley, and the High Council quietly observed Yuya's Duel against Jack Atlas. Declan suggested that Riley enter the Friendship Cup and mentioned that he had no problem letting Riley live with the other Duelists. He later watched Gong's Duel against Crow Hogan and when the Council asked that why Gong didn't use an Action Card, Declan explained that this was Gong's Dueling style. Riley was set to go up against Shinji Weber, but to Declan's surprise, Riley refused and fled to Yuya's room. Preventing Moon Shadow from recapturing her, Declan followed Riley to Yuya's room. Yuya protectively defended Riley, angrily asking why she was here. Declan admitted that he had forced Riley to come, claiming that she was necessary in the fight against the Academy. He then asked Riley why she didn't want to fight, and to his surprise, Riley claimed that she didn't want to fight Shinji since the Commons citizen had showed him kindness previously. Declan then had Moon Shadow Duel Shinji in place of Riley, and he expressed pride in Riley's sense of self-developing. The two siblings left Yuya's room together, their bond restored. Declan apologized to the High Council for the transition, explaining that Moon Shadow was a skilled Duelist. When Moon Shadow intentionally lost the Duel, Declan revealed to the shocked Council that he knew that they would ship Moon Shadow to their Underground Labor Facility, and that he had planned for the occasion.

At the end of the Duels of the first day, two of the Lancers had lost, while Zuzu and Celina had both won. Declan explained to the Council that Zuzu was also from the Standard Dimension and was equal in skill to the Lancers, so the Council decided to treat the results of the day as 2-2 for the Lancers. They invited Declan and Riley to dine with them and a guest; Jack Atlas himself. Jack expressed regret that Riley had been withdrawn; as the look in Riley's eyes had reminded him of himself at a young age. He explained his history to Declan and praised Riley, something that Declan thanked him for.

The second day of the Friendship Cup began with Yuya Dueling Officer 227. During the Duel, 227 Fusion Summoned, causing Declan to immediately suspect that there was a rat somewhere in the Synchro Dimension. When Yuya brutally won the Duel, Declan remained silent. While watching Yugo and Sylvio's Duel he then learned who Yugo was. . Moon Shadow escaped from the Underground Labor Facility and informed Declan and Riley of the rumors that Jean was from another dimension.

When Dennis was confirmed to belong to Duel Academy, the High Council demanded Declan to explain why their enemy was with them. Declan responded that he had considered the possibility that they had been infiltrated, but asked the Council if the same might not be true of their world. After Shay defeated Dennis, Declan prepared to interrogate Dennis, but the High Council stopped him and explained that they wanted to keep the City away from the Interdimensional War.

Friendship Cup Finals
When Roget revealed his plans to make the City into his kingdom, Declan asked if he plans to rebel against Duel Academy. Roget then offered him an alliance since they both shared a common enemy, but Declan refused saying that he has no benefit to ally with someone like him. He later watched Zuzu's Duel against Sergey Volkov and when the High Council believed that Zuzu had the Duel won, Declan corrected them, knowing that Sergey was up to something. The council later demanded him to explain why he rejected Roget's offer but Declan remained silent.

Declan watched Yuya's Duel against Shinji and was shocked when Yuya Pendulum-Synchro Summoned "Enligtenment Paladin" and won with it. When the council complained why Roget was praising Yuya after he won, Declan told them to not to worry as Yuya wasn't that foolish, because he is Yusho Sakaki's son. When Riley asked where Yuya was Declan told him that Roget was not going to cancel the tournament and Yuya would have to participate in another match eventually. He also noted that Roget was trying to win Yuya over to his side.

He later watched Yugo Duel against Celina. After Yugo defeated Celina, he then told Riley to rescue Celina because Roget wouldn't send her underground and wanted Riley to stop him. Declan then Dueled the Security officers in the Council Chamber and defeated them all. The Council, save White Taki, noted that Declan could have done so any time he wished and Declan replied that he was waiting for the right moment. Declan then told Roget that he wouldn't give Zuzu Boyle and Celina to him or Duel Academy. He also noted that Roget's plan had failed judging from the look on his face. After that he mused to himself that Riley was capable of rescuing Celina and hoped that Zuzu wouldn't fall into Roget's hands.

He then observed Yuya's Duel against Crow Hogan. While watching the Duel, he noticed the Obelisk Force flying in the sky.

After Jack Atlas defeated Sergey Volkov in the altered semi-final match, Roget declared martial law on the City. Declan took this opportunity to leave the Capital Building, despite the protests of the High Council and he used the effect of "D/D/D Duo-Dawn King Kali Yuga" to destroy the web of Trap Cards that Security had Set, inadvertently saving Riley, Shay, Sylvio and Chojiro. He smiled as Riley happily hugged him. Declan denounced the High Council for merely observing events, claiming that observers had no right to dictate the future. Predicting that Yuya would be heading to the Duel Palace, Declan directed Shay to take the Lancers to the Duel Palace on Shay's "Raidraptors", where they found Yuya, Gong, Sam, Frank, Amanda and Tarren on the bridge to the Duel Palace about to be attacked by Commons who had mistaken them for retreating Topsiders. Once he was in the stadium, Yuya challenged Jack to a Duel. Declan assisted his Lancers and Chojiro in fending off Security when they stormed the Duel Palace, defeating a "Goyo King" with "Caesar Ragnarok". When Yuya showed concern for Riley, Declan held him back and they watched as Riley defeated her opponent. In the middle of the Duel, Declan told his fellow Lancers to come with him, leaving Chojiro, Frank, Tanner and Amanda behind to watch the Duel between Yuya and Jack as he and Riley then headed to Security headquarters.

With Riley and Moon Shadow at his heels, Declan arrived just in time to prevent Roget from escaping with Zuzu. He revealed his correct predictions on what Roget planned to do with Zuzu and Duel Academy and his own future, showed his contempt for him, and demanded Roget release Zuzu and beg for the City's forgiveness. Roget refused and ran to the other direction. Declan ran after him and entered a room. Realizing it was some sort of trap, he pushed Riley back and let himself be locked up with Roget, who had hidden Zuzu away somewhere. The maniacal former director of Security claimed that he would also capture Declan and bring him to Duel Academy, but Declan declared that when he entered Duel Academy, he would destroy it. The two then began Dueling, with Roget Fusion Summoning "Ancient Gear Howitzer" and Declan taking 1000 damage from its effect. Declan Fusion Summoned "D/D/D Dragonbane King Beowulf" and Pendulum-Synchro Summoned "D/D/D Cursed King Siegfried", using them to defeat Roget in an OTK, but Roget vanished when he was slammed into the walls. A new Roget appeared and joined the Duel, taking the 2000 LP intrusion penalty, and he claimed that he would revive like a phoenix and that the Duel would continue for eternity until Declan surrendered. The Lancers, Jack, Crow and Sora then arrived, but Declan kept them back and he set up a controlled loop with "D/D/D Chaos King Apocalypse", "D/D Savant Nikola", "D/D Savant Thomas" and "Dark Contract with the Swinging Abyss", which would allow him to deal 2000 damage each turn to Roget before he could do anything, thus rendering Roget as the one who would be eternally defeated. Roget angrily accused Declan of causing Roget to take the fall for him and accused Declan of cooperating with the Professor, who he revealed to the Lancers to be Declan's father, of attempting to subjugate the four dimensions. Declan countered by pointing out that the accusation made no sense if Roget was fleeing with Zuzu, and Roget fled, but was stopped by Jack and Crow. Roget then attempted to plunge the entire City into the void and Declan saved Riley from being pulled into the wormhole, but Roget, Zuzu, Yuya, Gong, Shay, and Sylvio were all pulled in and sent to another dimension. Declan grimly noted that they had no way to find the lost Lancers.

Heartland City
The Lancers wished to assist in the rebuilding of New Domino City, but Shinji and Chojiro told the Lancers that they could handle it and they suggested that they do their duty. Joined by Sora and Crow, the Lancers traveled to the Fusion Dimension and procured a speedboat to go to Duel Academy; Declan taking the wheel. They came across Captain Solo's ship, where Gong and Sylvio were captured and Yuya was falling from the mast. Declan accelerated quickly enough for Crow to catch Yuya with "Assault Blackwing - Raikiri the Rain Shower". When Solo used "Grand Naval Battle" to attack the speedboat, Declan dodged the cannon fire. After Yuya and Sora defeated Solo, Declan told Yuya that Sector Security had disbanded and New Domino City had become truly equal. He told the Lancers that they would go to Duel Academy to rescue Zuzu and Celina and destroy Leo's plan, and end everything by the hands of the Lancers. Declan was surprised when he learned that Yusho Sakaki was heading toward Duel Academy and he revealed to the Lancers that he went there three years ago and that he had asked Yusho for help after learning about his father's plan. However, Yusho used his Interdimensional Travel Device without telling anybody and disappeared. He also met Celina and after meeting Yuya and Zuzu three years later, a memory resurfaced from Leo's research: Revival Zero. In it contained a file with two people, Celina and Yuri, who resembled Yuya. He told Yuya that his theory about the Four Dimensions and the four girls being connected to the Arc Area Project was probably correct, and he deduced that Revival Zero was another part of Leo's plan and that Yuya was probably involved given Yuri's involvement. Declan told the Lancers to believe in their friends' strength and fight to the very end. After arriving on the island, Declan prepared to climb the cliff to charge into his father's lab. After climbing up the mountain, they were surrounded by Duel Academy students before utilizing their plan to spilt two groups with Declan, Riley, and Sora continuing to the lab while Yuya's group Dueled the students.

Declan reunited with Yusho and told him he wouldn't let him get ahead of him as he had three years ago. Declan declared that they had to destroy Duel Academy, but they were confronted by Yuri. He, Yusho, and Riley managed to pass the fence that Sora lowered to block Yuri off while Alexis Rhodes faced him. He told Yusho who Yuri was and they continued to the Professor's chambers. They were confronted by Duel Academy students, who took them directly to the Professor.

Duel Academy
Declan was forced to calm Riley down after she reacted in fear to Leo. He listened to Yusho and Leo discuss creating the ARC System together, and he was surprised to learn that the came from the Original Dimension. He learned how Leo created the ARC System, which caused Duel Monsters to evolve but led to the world's ruin at the hands of a Duelist named Zarc. Declan realized that something was wrong after Riley sensed a powerful anger. He then learned how Zarc fused with his four dragons to become the Supreme King Dragon Zarc. The story scared Riley again, and Declan tried to calm her down by reassuring her that it was a myth, but Leo retorted that it was true.

Declan was pleased to see Yuya make it to the inner chambers and he wondered what Leo meant by bringing back the truth. To his surprise, Leo summoned Zuzu and her counterparts to the room and sealed the Doktor into a card. Leo explained that he created four cards that could defeat Supreme King Dragon Zarc, but they were stolen by his daughter Ray, who defeated Zarc and split the dimensions. Declan was surprised to learn that Leo had a daughter. Leo claimed that Ray was somewhere in the Four Dimensions, and Declan revealed that Leo began immersing himself in his research, ignoring his family and causing his mother to change. He explained that he had intended to find out what Leo was doing and punish him for hurting his mother. Leo explained that he believed Ray had survived and found evidence in Duel Academy. Declan deduced that Leo meant Celina and he learned that Ray had split into Zuzu, Celina, Lulu Obsidian, and Rin. To his surprise, Leo revealed his intention to resurrect Ray, and Declan wondered how sealing people into cards could bring her and the world back. Leo revealed ARC-V, and he explained that it would bring the back both the fifth dimension and Ray. Declan remembered reading the Revival Zero file that had Celina and Yuri in it and he wondered if Yuri was the Fusion Dimensional fragment of Zarc as Celina was of Ray. To Declan's shock, Leo asked him to help defeat Yuya because he was a reincarnation of Zarc and Declan's sister had sacrificed and split herself to defeat Zarc. Declan refused, disbelieving that Yuya could be a demon as Yuya's Dueltaining had brought smiles to people and stopped Duel Academy's invasion of the Xyz Dimension. Declan blamed Leo for disturbing the peace of the Four Dimensions and for bringing fear to people for one world, and he teamed up Yuya and challenged Leo to a Duel.

Accepting Yuya's help, Declan Pendulum-Fusion, Sychro, and Xyz Summoned "D/D/D Flame King Genghis", "D/D/D Gust King Alexander", and "D/D/D Wave King Caesar", then used "Dark Contract with the Monopoly Seal" to prevent Leo from using the same Summoning methods as long his monsters remained on the field. Leo tried to convince him to join him again, but Declan still refused, claiming that if Leo didn't fuse the dimensions, Zarc wouldn't revive and the Four Dimensions would be at peace. He declared that Yuya was his friend and accused Leo of accelerating Zarc's revival. To his surprise, Leo Pendulum Summoned, and his "Spirit Tech Force - Pendulum Governor" destroyed "Genghis" and prevented "Gust King Alexander" and "Wave King Caesar" from attacking. Leo asked Declan to join him again, but Declan preoccupied by Yuya's awakening and told him not to lose it. He watched Leo take damage from the effect of Yuya's "Performapal Gatlinghoul".

Leo survived the effect and ordered the energy input of ARC-V to be increased. Declan compared Leo to Zarc, criticizing him for burning through the sealed people like fuel and vowed to protect the Four Dimension's peace. He Tributed "Gust King Alexander" and "Wave King Caesar" to Summon "D/D/D Doom King Armageddon" and used the effect of "Wave King Caesar" to add "Illegal Dark Contract with the War God" to his hand, then used it to increase the ATK of "Armageddon" and lower that of "Pendulum Governor" to bypass its effect. He told Yuya to hang on and attacked, but Leo used "Dis-swing Fusion" to take control of "Doom King Armageddon" and Fusion Summon "Master Spirit Tech Force - Pendulum Ruler". Declan warned Yuya to be ready for Leo, but to his horror, Leo gave Declan's "Armageddon" the honor of killing Zarc's alter ego. The sight of Yuya going for an Action Card convinced him that Yuya was in control even though Zarc was overflowing from inside him. He reassured Yuya that he would save him after they toppled Leo's ambitions. To Declan's surprise, Yuya and Yuto managed to make brief contact with him before screaming that they would become one. Leo declared that Yuya no longer existed after he Xyz Summoned "Dark Rebellion Xyz Dragon", but Declan believed Yuya that was still there. He told Yuya not to be swallowed by Zarc and reminded him that he was Dueling for peace and bringing smiles to the future, but was surprised to see a dragon's silhouette form from Yuya's dark aura. Yuya Xyz Summoned "Odd-Eyes Raging Dragon" to destroy "Pendulum Ruler" and "Armageddon", and took damage when Yuya destroyed "Armageddon" due to the effect of "Dis-Swing Fusion". Yuya activated the effect of "Performapal Nightmare Knight", which would defeat Leo and Declan as they had both taken battle damage. Fortunately, Yusho restrained Yuya "Binding Swords of Impact", which terminated the Duel. Yusho passed Riley to Declan and explained that he was going to prevent Zarc's revival.

Watching Yuya trying to free himself, Declan demanded that Leo turn ARC-V off, but Leo entered the machine and sealed off the throne room. He woke Riley in order to stop ARC-V but to his surprise, Riley talked with Ray about her revival. He told Riley that if the Four Dimensions become one again everybody might disappear. Riley asked him if he didn't want to revive his real older sister, and in response Declan asked Riley if he was fine with everyone vanishing. Riley challenged Declan to a Duel, and Declan Fusion, Synchro, and Xyz Summoned "Genghis", "Alexander", and "D/D/D Boulder King Darius" and sealed most of Riley's Deck with "Dark Contract with the Eternal Darkness". To his surprise, Riley continued to Duel and she stalled with "Film Magician", but Declan used the effect of "Darius" to destroy "Film Magician", and as he attacked he reflected on how he gave Riley light when she grew up in hellish darkness. Riley used "Damage Banish" to remain in the Duel, and as Riley copied Declan's Trap and destroyed it, Declan was surprised to see Ray's spirit appear next to Riley. Riley Fusion Summoned "C/C/C Sonic Halberd of Battle", a monster Declan never knew Riley had. As Riley destroyed his monsters and prepared to attack him directly, Declan reflected on how Riley had always copied other Duelists, but her sense of self was now awakening. As "Sonic Halberd" launched a final attack, Declan wondered where he had gone wrong and created a monster.

He used "Miracle" to negate the attack, and told Riley that it was fine if she believed in carrying out Ray's will. Regardless, as they opposed Declan in protecting the Four Dimensions and defeating Leo, Declan vowed to defeat Riley and keep moving forward, and he brought out "D/D/D Knowledge King Tomb Conquistador". He defeated Riley and ran to go check on her as Yusho was blasted into the chamber by Yuri. Yuri revealed that he had absorbed Yugo, and Declan tried to protect Yusho, but Yuri blasted him away. Riley revealed that Zarc's revival was close as Declan watched Yusho and Yuri's Duel. He was surprised to see "Performapal Sky Magician" and observed Yusho's Dueltaining calming Yuya's heart. He was pleased when Yusho countered Yuri and sent "Super Polymerization" back into his Deck. Yuya explained Yusho's tactics to Declan, and he realized Yuya was back to his original self. As Yuri locked Yusho down with "Ivy Bind Castle", Riley wondered who Yuya was talking to. Declan explained that Yuya was talking to Yuto in his mind, a sign that he wasn't engulfed by the darkness yet. He was surprised when "Sky Magician" escaped from "Ivy Bind Castle" and Yusho destroyed Yuri's monsters. Declan wondered what Yuri when he started to act confident, but was surprised when Yuri revealed that "Super Polymerization" wasn't his trump card, but "Transcendental Polymerization" was. Yuri used it to Fusion Summon "Starving Venom Fusion Dragon" and Synchro Summon "Clear Wing Synchro Dragon", causing Yuya to begin screaming in pain. As Yuri prepared a combo using "Starving Venom", "Clear Wing", "Flower of Destruction", and "Fruit of Destruction" to damage Yusho, Declan prepared to join the Duel, but Yusho told him not to interfere. Yuri defeated Yusho and sealed him into a card, and Yuya broke free from "Binding Swords of Impact".

Declan tried to stop Yuya from being influenced by the darkness, but Yuya blasted him away and Declan warned Yuya not to approach Yuri carelessly. As he watched Yuya writhe in pain and the dragons resonated, Riley explained that Ray had told him that Zarc's revival was inevitable. Declan noted that Yuya was fighting for them to avoid it. Eventually, Yuya and Yuri Summoned all Four Dimension Dragons blasting Riley back when they resonated, and Declan caught her. He told her that Yuya was still fighting for the world. Distortions in space began occurring, and Declan realized that ARC-V had started the dimension integration. Yuya came close to giving into the darkness, and blasted Declan back when he called out to Yuya. He told Yuya not to lose his self or waste Yusho's ideals. Yuya started to move near Yuri, and Declan desperately yelled at him to stop but to his surprise and relief, Yuya was simply Setting a card. Leo trapped them in the Isolation Dome, and as Declan ran to check on Yuya Riley cried that she'd lost contact with Ray. Sora and Aster Phoenix arrived and he explained what had happened. Declan, Riley, Sora, and Aster attempted to join the Duel to protect Yuya, but Yuri blasted them away. Aster asked if Yuya had any chance of winning, and Declan assured him that Yuya had a card to turn the Duel around. However, Yuya began tearfully apologizing to his father after he used "Farewell", and Declan learned that Yuya had to discard "Smile World". Declan reassured Yuya that he wouldn't become a demon, believing in him to save the world. He told Yuya to focus on winning for now, and watched him take control of "Dark Rebellion" and "Clear Wing", and he realized that Yugo and Yuto were supporting Yuya to defeat Yuri, and Declan encouraged him to win as well. After Yuya defeated Yuri, Declan noted that ARC-V was the final obstacle to deal with. As Yuri told Yuya to absorb him quickly, Declan retorted that Yuya would not absorb him to become a demon. Declan realized that something was wrong when Yuya reminded him that he had told him to win, just as Zarc had been encouraged in the past, and Yuya fell under control of the darkness and summoned "Astrograph Sorcerer" to fuse with the Four Dimension Dragons. He ran outside with Riley, Sora, and Aster to find the world floating in pieces as the Supreme King Dragon Zarc alighted in front of them.

Zarc introduced himself, and Declan wondered where he was to have Summoned the Supreme King Dragon. As the Supreme King Dragon Zarc began to attack, Riley wondered where Leo was. Declan reassured him that Leo would have already evacuated, but Riley explained that Ray was calling Leo to hurry and revive her since only she could defeat Zarc. Riley ran back inside Duel Academy, and Declan pursued her. While walking through Duel Academy, Declan noted that the dimensional integration was becoming more intense. He and Riley saw the citizens of Heartland and New Domino City walking through them in midair, and Declan wondered if it was because of the dimensional integration. As they continued to walk through Duel Academy, Declan learned from Riley that Ray had told her that Leo was trying to split the world again using the four cards that had revived instead of her.

Riley explained that Leo planned to battle Zarc with the cards, and Riley asked Declan to save Leo so that Declan would smile again. Declan admitted to himself that he wouldn't be able to smile if he was grieving, and he agreed, telling Riley stay where she was. He joined the Duel against Zarc right after Leo did, receiving an intrusion penalty. Declan explained to Leo that he wasn't the only person responsible for Zarc becoming evil, claiming that he was as well. He clarified that society itself had created Zarc. To his horror, Leo's attempt to defeat Zarc with the four cards failed, and Zarc defeated Leo, blasting him through the air. Declan managed to catch his father and drag him to safety, before preparing to face Zarc alone.

When Leo was about to give up, Declan told him there was still hope as everyone that Dueled Zarc believe Yuya's soul is inside of him. He received Yuya's pendant from Sylvio and told Leo the reason Zarc could use Pendulum was because of Yuya's soul still exist. Declan reminded Yuya that he was the founder of Pendulum Summoning and how he studied him since its first usage. Declan told Yuya how everyone was influenced by him since he moved the hearts of many people, so should be able to overcome Zarc's evil heart. He showed Yuya his Dueling by Pendulum-Xyz, Synchro, and Fusion Summoning "D/D/D Wave High King Executive Caesar", "D/D/D Gust High King Executive Alexander", and "D/D/D Flame High King Executive Genghis". Declan used the effects of "Executive Alexander" to double its ATK and "Executive Genghis" to negate the effects of "Supreme King Gate Zero" and "Infinity". Zarc tried to use the effect of "Supreme King Servant Dragon Odd-Eyes" to revive his "Supreme King Servant Dragon Darkwurms", but Declan negated it the effect of "Executive Caesar" and add "Darkwurm's" ATK to "Executive Caesar" and "Executive Genghis". Declan attacked "Supreme King Dragon Zarc", which couldn't be destroyed due to Fusion, Synchro, and Xyz Monsters being in the graveyard or banished, and dealt massive damage to Zarc. Declan was surprised when Zarc told him he was the true founder of Pendulum, which was born for his revenge against Ray and learned that when Ray split the world into four, Pendulum was born at that moment and Zarc resurrected with it.

When Zarc used the effect of "Supreme King Servant Dragon Clear Wing" to destroy "Executive Alexander", Declan used the Pendulum and monster effects of "D/D Savant Dirac" to Special Summon it from his Pendulum Zone and reduce the damage to zero. When Zarc used "Supreme King Servant Dragon Dark Rebellion" to destroy "Executive Caesar", Declan used the Pendulum and monster effects of "D/D Savant Schrödinger" to Special Summon it from his Pendulum Zone and reduce the damage to zero. Zarc used the effect of "Supreme King Servant Dragon Starving Venom" to steal the effect of "Executive Alexander" to double its ATK and attacked "Executive Genghis", but Declan used "Schrödinger's" effect again. Declan was almost defeated when Zarc used "Odd-Eyes" to attack "Dirac", using the second effect of "Staring Venom" to grant the ability to inflict Piercing Damage and "Odd-Eyes" to double the damage. He was then defeated by Zarc and saved by Moon Shadow as Riley intruded the Duel. He called out to Riley but was surprised to learn that Ray was in control of Riley's body. He watched Ray and Riley destroy all of Zarc's dragons with the Natural Energy Cards and told Zarc that he's both of Yuya & Zarc since his heart was constantly swinging like a pendulum. When Yuya managed to regain control, Declan tossed his pendant back to him and watched his siblings defeat Zarc. But, when the world was about to split into four again, Declan noted that Zarc would reincarnate again like before.

Following the battle against Zarc, Declan told his mother how the Standard Dimension was reborn into the "Pendulum Dimension" and that everyone could Pendulum Summon as usual now. When asked about the baby, Declan told Henrietta to feel at ease. He then sent invitations to Yuya, Gong, Sylvio and Moon Shadow for the Junior Arc League Championship and told them they would Duel in a Battle Royal first. During the Duel, Declan had Moon Shadow assist in helping them remember their past and not to hold back. When Henrietta wondered if this would help bring back Riley's smile, Declan was sure as Yuya was the only one. He told Moon Shadow not to show mercy and agreed with Yusho that Yuya was at his limit if he lost. He also told Henrietta to give up on Riley's smile. When Yuya regained his memories of Zuzu, Declan revealed that Riley expelled her own soul in order to seal Zarc's soul and was reborn as a baby without a smile due to the dimensional spilt. Nevertheless, he still believed that Riley's smile would definitely return since they took back their memories for that reason.

When Yuya, Gong, and Sylvio wondered why Riley was a baby, Declan revealed that the Natural Energy Cards purified Riley, turning her back into a baby and due to Zarc splitting the dimensions Ray and Riley were reborn as four different babies. But, Riley absorbed Zarc's soul to save Yuya and the four dragons returned the dimensions back into four after ARC-V unified them. Declan believed that the reason Riley didn't separate into four people was to quickly seal Zarc's soul, which took his smile away. He then revealed the reason for reenacting the Arc League Championship was to return Riley's smile and save the world through Yuya's Dueltaining. He also states if they don't do it, Zarc will resurrect and the world will be at risk again. Declan also believe Zuzu is safe and her being saved or not is up to Yuya. When Yuya used "Performapal Signal", Declan noted he'll lose if its not a Performapal monster. He was surprised Yuya and Gong were able to deal damage to each other in the Ancient Ruins. Near the end of the Duel, Declan left Leo Corporation to go to the arena and announced that Yuya is not promoted into the Senior class yet. He told Yuya and Gong that the Battle Royal was done as a continuation for the Arc League Championship due to Duel Academy's invasion and will only permit one Duelist from the Lancers advance to the Senior class. He took Yuya to the dimensional corridor gate, remnants of ARC-V connected with the Four Dimensions, to send him the Xyz Dimension.

He watched Yuya and Dennis' Duel, thinking Yuya would use the effect of "Xiangsheng Magician" to change "Performapal Sword Fish's" Level and overlay it with "Performapal Miracle Mirrorabbit to bring out "Dark Rebellion". However, he surprised when Yuya decided to battle instead and watched Yuya defeat Dennis. When Yusho when wondered who Jack Atlas was, Declan told him he is a Synchro Dimension Duelist who had a fierce Duel with Yuya and helped Duel against Zarc. When Jack demanded Yuya to use the Four Dimension Dragons in their Duel, Declan agreed and told Yuya that he needs his Dueltaining to get Riley to smile and purify Zarc's soul. He even told Yuya his Duels with Dennis and Shay didn't make Riley smile and prepared a stage for his next Duel, Dimension Highway, which spreads throughout the Four Dimensions. Declan also told Yuya that he wanted to show the people in every dimension the Ultimate Turbo Duel and make Riley smile. While watching the Duel, he learned that Yuya is afraid of summoning the four dragons was because something might happen to Zuzu, Celina, Lulu, and Rin as well and started to worry. When Yuya Xyz Summoned "Dark Rebellion", Declan noticed Riley reacting to it. After all four dragons were summoned, Declan learned from Yusho that the true fun was about to begin.

When Riley's dark aura disappeared, Declan wondered if she was feeling Yuya's Duel with Zarc's dragons. He went to the stadium after Yuya won and told him that Zarc's soul nearly awakened after the four dragons were summoned, but Riley hasn't disappeared inside yet. He also told Yuya that he had been promoted to the Senior division for completing the tasks and that the Senior Duelist was entitled to challenge for protest. When Yuya challenged him, Declan, figuring that would happen, announced to everyone that he would protest Yuya and be his opponent. Declan chose the action field to be "Performance Colosseum" and told Yuya to surpass if he wished to save Riley and the world. He watched Yuya Pendulum-Fusion, Synchro, and Xyz Summon, "Odd-Eyes Pendulum Dragon", "Starving Venom", "Clear Wing", and "Dark Rebellion" and wanted to make Yuya's confidence was genuine or not. He Pendulum Summoned his three "Doom King Armageddon's" and attacked Yuya's dragons who countered each attack and destroyed one "Doom King Armageddon". After Yuya regained a memory of Zarc, Declan told him to try and make him laugh. He used "Dark Contract with Underworld Insurance" to draw three cards and used "D/D/D Fusion", "D/D/D Synchro", and "D/D/D Xyz" to use his "Doom King Armageddon's" as materials to Fusion, Synchro, and Xyz Summon "D/D/D Superdoom King Purplish Armageddon", "D/D/D Superdoom King Whitest Armageddon", and "D/D/D Superdoom King Darkness Armageddon". He then used the effect of "Darkness Armageddon" to attach "Doom King Armageddon" to itself as an Overlay Unit from his Extra Deck. Declan then told Yuya that he built an iron wall for Yuya to break through if he wants him to laugh from the bottom of his heart.

Riley Akaba
While Declan and Riley aren't as close compared to other pairs of siblings in the previous series such as the Kaiba brothers, they still are shown to care about each other. Although they aren't related biologically, Riley was encouraged by Declan's compliments after her victories during the Arc League Championship and was allowed to join the Lancers despite not being at the Battle Royal. When some of the Lancers complained about Riley joining, Declan quickly defended her and said that she would not be a hindrance, and constantly shows his confidence in Riley's skills.

When the Lancers were split in the Synchro Dimension, Riley cared about nothing but finding Declan as soon as possible, and Declan requested that Moon Shadow find Riley and later to take her place in the Friendship Cup. When Riley showed signs of developing a "sense of self", he was shown to be proud and seemingly resents his mother's plan of using Riley as a mindless puppet and even when Riley was first brought to them he felt very uncomfortable for making a very traumatized child battle against Duel Academy.

Declan's confidence in Riley was enough to entrust her with rescuing Celina from Sector Security and later showed no surprise when Riley defeated three Sector Security officers at once.

Leo Akaba
When Declan encountered Leo in the Fusion Dimension he was suspicious of his father's ambitions and later founded the Lancers to fight against him. He claimed that Leo doesn't care about him when Shay planned to use Declan as hostage. Declan was furious when he heard about the suffering that his father had caused.

Shay Obsidian
When Shay Obsidian and Declan met face to face, rather than Dueling Shay, Declan decided to form an alliance with the Xyz Dimension Duelist. However he frequently has to keep Shay under vigilance so his recklessness doesn't affect his plans.

Celina
When Declan met Celina in the Fusion Dimension, he tried to help her escape without hesitation. Despite knowing Celina's allegiance to Duel Academy, Declan trusts her and plotted to have her to join the Lancers knowing she wouldn't betray them. Celina likewise accepted, agreeing to help Declan combat the Fusion Dimension and supported his plan to go to the Synchro Dimension to recruit allies.

Yuya Sakaki
According to director Katsumi Ono, Declan sees Yuya as an object of study. During their first Duel Declan and Yuya were respectful to one another and Declan revealed that he respected Yuya's father. During their second Duel however, Declan showed indifference towards Yuya's feelings, disregarding them as a weakness even though he acknowledged the strength that they gave him, and instead used them to manipulate him and push him to become stronger. During their Duel, he refused to tell him the truth about Zuzu's disappearance and only told him the truth just as they were about to leave for the Synchro Dimension, where Zuzu was (and even then, likely largely due to Celina figuring this out herself and so Yuya would be more determined to go there). Despite his attitude, Declan continues to respect Yuya and trust him as shown when he assured Riley that Yuya won't be foolish enough to accept Roget's offer. He also inadvertently saved Yuya from becoming Roget's puppet, where if he were not been able to defeat Sector Security in the High Council's room on time, Roget's plan of brainwashing Yuya would have remain unaltered, and Yuya would have undoubtedly become a soldier of Roget.

Although Yuya tends to disagree with Declan's methods, he knows that at heart Declan is a person he can trust. This is demonstrated when Roget attempted to use the father-son relationship of Leo Akaba with Declan as "proof" that Declan was using the Lancers for his own ends. However Yuya defended Declan causing him to smile.

Likewise when his father Leo requests Declan to join him to defeat Yuya since he is a part of Zarc and by proxy a threat to the world, Declan refuses and defends Yuya as his own person who genuinely seeks to resolve conflicts and bring smiles to people with his Dueltaining, and promptly challenges his father to a Duel along with Yuya, in which Declan stands by Yuya's side despite Leo's warnings affirming he truly thinks of Yuya as a friend.

Any hostility Yuya may have felt towards Declan appears to have vanished after their reunion in the Fusion Dimension as they have co-operated well and affirmed each other as friends.

Yusho Sakaki
Three years prior to the events of ARC-V, Declan and Yusho agreed to meet Leo Akaba and attempt to talk him out of the Arc Area Project. When Declan found out Yusho was on his way to Duel Academy, he knew right away that Yusho was still following their plan. When the two reunited, they greeted each other in good terms, and worked together to go confront Leo and stop the project.

Deck
Declan uses a "D/D" Deck, which utilizes the "Dark Contract" archetype of Spell and Trap Cards to easily gather the necessary cards to perform Fusion, Synchro, Xyz, and Pendulum Summoning of his "D/D/D" monsters, using cards such as "Contract Laundering" and "D/D/D Human Resources" to avoid the negative effects of his cards.

Declan's usual tactics involves multiple combos per turn through methodical and lined-up patterns of the same cards. Declan is capable of using Fusion, Synchro, and Xyz Summoning and his mastery of them is in the opposite order of that of Yuya Sakaki. While Yuya first acquired Pendulum Summoning and then acquired the other three Summoning methods, Declan mastered them first and then acquired Pendulum Summon. The opposite order of the mastery of the four specialized Summons further highlights Yuya and Declan's rivalry.

Over the course of the series, some of the cards used by Declan have been changed with regards to their effect or type. This is the list of cards which he used previously.