Mokuba is the younger brother of Seto Kaiba. He is initially perceived as a spoiled brat, with vindictive and malevolent behavior. He was a champion at Capsule Monster Chess, but cheated in many of the games he played. His negative traits are partly credit to the neglect he gets from his older brother, whom he desires to prove his worth to. His behavior improves as he befriends Yugi Mutou and his companions and as Seto grows to truly appreciate him.
At a young age, Mokuba and Seto were orphaned. Their mother died when Mokuba was born. Their father died in an accident when Mokuba was three. Their relatives used up their inheritance and left them at the city orphanage. Due to the trauma of their losses, Seto came to tell Mokuba never to trust anyone.
When the former head of Kaiba Corporation, Gozaburo Kaiba, arrived at the orphanage to adopt an heir, Seto saw the chance to challenge him to a game of chess, with Gozaburo forced to adopt Seto and Mokuba should he win. Mokuba was five at the time. Seto had won by cheating.
As Seto usurped Gozaburo for the presidency of KaibaCorp, the young gamer descended into evil, and grew to neglect Mokuba more and more, leading to Mokuba to go to many lengths to earn his brother's affections.
Mokuba led a gang of followers his age. With them, he planned to beat Yugi in a game to get revenge for him beating his brother in Duel Monsters and inflicting a Penalty Game on him. Mokuba got Old Man Dentures to rig his Capsule Monster Chess coin machine. He and his gang later arrived at Dentures' store to meet Yugi at the coin machine. After introducing himself as Seto Kaiba's brother, to Yugi, he has his followers seize Yugi and forced him into a game. They proceed to take Dentures coin machine. Dentures acted mad at first, but allowed them to carry on after Mokuba dropped a large amount of money in front of him.
The gang took Yugi and the dispenser to their hideout, where they forced him to play Capsule Monster Chess against Mokuba. As Mokuba hoped, Yugi changed to Dark Yugi for the game. The rigged dispenser gave Yugi mostly low-level monsters and Mokuba mostly high-level ones. They accepted the stakes that if Mokuba won, he got to cut off one of Yugi's fingers and if Yugi won, Mokuba would have to play a Penalty Game. Dark Yugi made the game a Shadow Game. Throughout it, he sacrificed his monsters to draw Mokuba's into a diagonal line and used his last monster "Torigun" to take Mokuba's remaining four monsters all out at once. For Mokuba's Penalty Game, he believed that he was trapped inside a giant Capsule Monster Chess capsule. Before the capsule had Mokuba fully engulfed, he warned Yugi that Death-T was coming.
During the planning of the theme park Death-T, Mokuba insisted on facing Yugi in one of the stages. Although Seto advised against it, they still went through with it. The two of them also made a bet over which stage Yugi would die in. Mokuba bet Death T-4, while Seto bet Death T-5.
When Kaiba's chauffeur brought Yugi and Jonouchi to their mansion, Mokuba met them in the car. He introduced himself as the vice-president of KaibaCorp and told Yugi his given name for the first time. He told Yugi not to worry as they would be giving him V.I.P. treatment for the grand opening of Kaiba's new secret project, but still maintained a suspicious air about himself. At the mansion, Mokuba invited Yugi and Jonouchi to a meal, featuring the best food in the world. However the meal turned out to be six servings of a meal a spoiled young child would like. He suggested that they take turns spinning the turntable that the food is on and eat the meal that lands in front of them. He lied that there wasn't poison in them and claimed that there was a treasure hidden in one. Jonouchi went first and Mokuba used a hidden switch in a syrup bottle to give him a poisoned meal. He admitted to lying and introduced the meal as a game of Russian Roulette Dinner. He forced Yugi to continue to win the antidote to save Jonouchi. Yugi switched to Dark Yugi, who figured out how Mokuba was cheating and managed to smash the syrup bottle, giving Mokuba a poisoned meal. Mokuba ate it and lost, handing the antidote over to Yugi. Mokuba fell over poisoned, while the butler rushed to his aid.
The next day, Mokuba prepared to face Yugi in another game of Capsule Monster Chess, during Death T-4. He confronted Seto about their bet on which stage Yugi would die on. He pointed out that Seto bet Yugi would die in Death T-5, meaning he bet Yugi would beat Mokuba. Seto stood by his choice, said that Mokuba cannot beat Yugi and reminded Mokuba of how he tried and failed to show him up by beating Yugi in a game before. Mokuba hung his head and said that he thought Seto would like him, if he beat Yugi. Seto yelled at Mokuba that there is no room for brotherly love in the world of gaming and until Mokuba realizes that, he would always be a loser. Mokuba stormed off to face Yugi, eager to prove Seto wrong.
Mokuba rigged the Capsule Monster dispenser again, but still lost. Seto, held no sympathy for his brother and subjected him to the "Experience of Death" Penalty Game, as per Death-T rules. However Dark Yugi returned and saved Mokuba from the grisly punishment. Mokuba was surprised that Yugi saved him and asked why. Yugi replied that he wouldn't have won, if he didn't have his friends supporting him. This caused Mokuba to reflect on his past and how Seto had changed since they left the orphanage. He wondered if Yugi could help Seto when he faced him in the next round.
To thank Yugi for saving him, Mokuba rescued Yugi's friend Honda, who had been trapped in the previous stage of Death-T. After Seto's defeat and subsequent "Mind Crush" Penalty Game, Mokuba explained the Kaiba brothers' history to Yugi and his friends. Dark Yugi told Mokuba that Kaiba would wake up again after he had reassembled the shattered puzzle of his heart and Mokuba vowed to wait as long as it took for his brother to return.
While Kaiba was in a coma after Death-T, Maximillion J. Pegasus and The Big Five tried to take over KaibaCorp. The Big Five came to Kaiba Manor one night to steal documents from a safe, but Mokuba swallowed its key in front of them. As a result Mokuba was kidnapped and brought to the Duelist Kingdom island, where he was kept under watch, particularly when he used the bathroom. Mokuba overheard that as part of Pegasus deal with The Big Five, Pegasus would have to beat Yugi in an official game of Duel Monsters to rebuild KaibaCorp's reputation. Yugi was also on the island for the Duelist Kingdom tournament, in which Pegasus hoped to defeat Yugi in the finals. Mokuba escaped his captivity to make an attempt to eliminate Yugi from the tournament in order to prevent him and Pegasus having an official Duel.
Mokuba disguised himself with a hat and bandanna. He challenged one of the tournament's participants to a Duel, but stole his Deck and Star Chips before the Duel began. Mokuba later encountered Dark Yugi, who had heard of the theft and was searching for the thief. He challenged Dark Yugi to a Duel, where they both wagered all their Star Chips, and Dark Yugi accepted despite knowing his opponent was the thief. Mokuba proved to be no match for Dark Yugi, so he tried to grab all the Star Chips and run. However Dark Yugi grabbed him by the wrist and removed his disguise. At first, Mokuba refused to return to Star Chips he had managed to grab and told Dark Yugi and his friends about Pegasus' plan to take over KaibaCorp. Dark Yugi promised that he would defeat Pegasus and Anzu convinced Mokuba to trust Dark Yugi. However Saruwatari stopped him returning the Star Chips, citing that it was against the rules to exchange Star Chips without Dueling.
Saruwatari grabbed Mokuba, confiscated his Star Chips and was about to take him back to captivity. Two of those Star Chips belonged to Dark Yugi, but Saruwatari refused to exchange Star Chips outside a Duel, so he arranged for Dark Yugi to face the Ventriloquist of the Dead. The ventriloquist claimed Seto Kaiba was dead and his soul resided in a puppet, in order to get revenge on Yugi. Mokuba was largely offended by the fraudulence. During the Duel, Saruwatari revealed his own identity to Mokuba, who remembered him as his bodyguard at KaibaCorp. Before the Duel was finished, he snuck away taking Mokuba with him.
When Kaiba woke from his coma, he traveled to Duelist Kingdom to save Mokuba. During his Duel with Dark Yugi, he imagined Mokuba getting sucked into his decaying "Blue-Eyes Ultimate Dragon". A vision of Kaiba's younger-self glared at him for losing Mokuba. This prompted Kaiba to put his life on the line, so that he could save Mokuba.
Kaiba challenged Pegasus to a Duel, for Mokuba's freedom, using Duel Disks. Pegasus knew better than to fight on his enemy's terms, so he showed Kaiba Mokuba's soulless body and said that he would only Duel with the Duel Disks if Mokuba would be his proxy and throw the disk for him. Kaiba refused to fight Mokuba, so he agreed to fight without the Duel Disks. Kaiba lost the Duel and also had his soul sealed in a "Soul Prison".
Feeling he owed Mokuba for saving him in Death-T, Honda asked Dark Yugi to try and free the souls of Kaiba brothers, while he took care of finding their soulless bodies. On the eve of the tournament finals, Honda searched Pegasus Castle and found Mokuba's body and managed to retrieve it with help from Ryo Bakura and Dark Bakura.
When Dark Yugi defeated Pegasus, Pegasus freed the souls of the Kaiba brothers and Sugoroku Mutou. Mokuba woke up in the bed that Honda had left him in. Kaiba woke up in Pegasus dungeon and an apparition of his younger self appeared and led him outside to Mokuba. On their reunion, the apparition of Kaiba held the "Puzzle of Kaiba's heart", which was now complete, before he vanished. Dark Yugi commented that Mokuba was the final piece of the puzzle. Before leaving the island on Kaiba's helicopter, Mokuba mocked Yugi and his friends for not having a helicopter of their own. However, when he saw that they had no way of getting home, he talked Kaiba into giving them a ride.
Mokuba stood alongside Kaiba at the announcement of the Battle City and helped him put on his Duel Disk as Kaiba explained some of the rules. During the tournament he worked as "steering committee chairman" and had the job of settling disputes.
Mokuba stepped in to adjudicate when he spotted Koji Nagumo trying to force a player he defeated to give him a second rare card or ¥100,000 through the ante rule. Mokuba explained that changing the ante after a Duel and betting money were both against the rules. He also ruled that the defeated player's wagered was a fair ante against Koji's "Hyozanryu". Koji was unwilling to cooperate. However Kaiba showed up and took over. Mokuba showed him Koji's "Hyozanryu", which Kaiba proceeded to rip up before challenging Koji to a Duel. To compensate, Kaiba wagered "The God of the Obelisk" and a briefcase of rare cards, the latter of which he allowed Koji to borrow during the Duel. Kaiba easily won and he and Mokuba left together.
When Kaiba was notified of Yugi dueling a Rare Hunter with an Egyptian God card, Kaiba and Mokuba both headed towards the Duel. Mokuba told Dark Yugi not to lose to the Doll, as he wanted Dark Yugi to Duel Kaiba later in the tournament. Mokuba was shocked by the high ATK values "Slifer the Sky Dragon" was achieving and asked Kaiba how Dark Yugi could defeat a monster with infinite ATK. Kaiba realized that Mokuba was wrong and warned Dark Yugi, telling him nothing was infinite; a clue which led to Dark Yugi winning the Duel. The Doll knelt motionless after the Duel, so Mokuba saw it as his duty to judge the ante. He handed Dark Yugi "Slifer the Sky Dragon" and the Doll's Puzzle Card, but told him not to get used to them, as they would soon belong to Kaiba.
Mokuba was shocked to see Kaiba immediately challenge Dark Yugi to a Duel, following the defeat of the Doll. However Marik Ishtar took control of the Doll again to say that he had put Dark Yugi's friends in danger. Dark Yugi left to find his friends and was soon followed by Kaiba, leaving Mokuba alone. Mokuba returned to a control center and following Kaiba's orders, used GPS to track Jonouchi's location. He then boarded a helicopter, which collected Kaiba and Dark Yugi after their Duel with the Masks of Light and Darkness. Mokuba asked Kaiba he was bothering searching for "that loser" Jonouchi. Kaiba explained that he was interested in seeing how Dark Yugi could possibly make use of the "power of unity" when forced to fight his friend, due to Marik's alleged brainwashing powers.
They tracked Jonouchi's location to Domino Pier. Mokuba saw Yugi's friends being possessed by the power of Marik's Millennium Rod and was disgusted by Marik forcing Jonouchi and Dark Yugi into a Duel, in which one of them would drown. He urged Kaiba to help them. However Mokuba was taken hostage by a Rare Hunter, who threatened to kill him if Kaiba interfered. Near the end of the Duel, Kaiba disarmed that Rare Hunter by throwing a card, which lodged into his hand. He then fought off the two Rare Hunters, saving Mokuba. After the Duel, Mokuba pulled Yugi out of the water and told him that Kaiba had saved Jonouchi.
Mokuba and Kaiba took the helicopter to Domino Stadium, to get to the location of the finals. After the Battle Ship arrived, Isono was going to refuse to let Anzu, Honda, Shizuka and Otogi board as they were not finalists. However Mokuba insisted that he let them on.
During Mai Kujaku and Dark Marik's quarterfinal Duel, Kaiba got Mokuba to use the orbital camera on their satellite to zoom in on Mai's Duel Disk, so that they could get an image of the text on "The Sun Dragon Ra".
The Dark Side of Dimensions
Mokuba took part in an excavation in Egypt to find the Millennium Puzzle buried with the Shrine of the Millennium Stone. Shortly after Seto called forth Obelisk the Tormentor in his Duel against Diva, the timer on the security system deactivated the laser field around the container holding the pieces. Mokuba absconded with the container via helicopter. Mokuba headbutted Diva away when he tried to stop Mokuba, but not before Diva swiped two pieces of the Millennium Puzzle.
Mokuba later appeared in the helicopter to carry Seto away after the latter demanded Yugi participate in his tournament for the puzzle pieces. After Seto had Diva captured, Mokuba interrogated him on how to retrieve the piece he hid in his Quantum Cube. Although Diva mocked him about their inability to extract it and about the whereabouts of the remaining piece, Mokuba calmly revealed that they had already determined the second piece to have been in the possession of his sister Sera, while also omitting that they already knew she had entrusted her piece to Yugi.
At the exhibition tournament, Mokuba witnessed Yugi’s matches with Diva and Seto, noting that the former had become another Dimension Duel like the one at the excavation site, and later experienced Dark Diva’s disintegration of the world. After the Pharaoh returned and helped Yugi defeat Dark Diva, the Quantum Cube was taken by Seto and used to engineer a Duel Dimension system in his space station. Mokuba advised his brother to be careful as Seto attempted to reach the Pharaoh in the afterlife. Before leaving, Seto left Mokuba in charge of KaibaCorp.
Mokuba was not shown with a Duel Monsters Deck of his own. However during Duelist Kingdom, he used a Deck, which he stole from a participant. The only cards shown in this Deck were "Crocodilus" and "Man-Eating Plant".
Capsule Monster Chess pieces
The first time Mokuba faced Yugi, he used the following Capsule Monster Chess pieces:
|Head Sucker||Gumbo||Cobrada||Dinosaur Wing||The Skull|
When he faced Yugi in Death-T, he used the following:
|Megaton||Nama Hargen||Big Foot||Zoid "M"||Armorsaurus|
Mokuba played the following games.
|Dark Yugi||Capsule Monster Chess (game)||Lose||24|
|Dark Yugi and Katsuya Jonouchi||Russian Roulette Dinner||Lose||26|
|Dark Yugi||Capsule Monster Chess (game)||Lose||34, 35|
|Dark Yugi||Duel Monsters (game)||N/a[n 1]||14 (73)|
- The Duel was canceled after Mokuba tried to grab the Star Chips and run away.
- Yu-Gi-Oh! Character Guidebook: The Gospel of Truth: Mokuba Kaiba's profile
- Yu-Gi-Oh! Duel 24: "Capsule Monster Chess"
- Yu-Gi-Oh! Duel 34: "Arena #2"
- Yu-Gi-Oh! Duel 26: "Russian Roulette"
- Yu-Gi-Oh! Duel 35: "Board Game Deathmatch"
- Yu-Gi-Oh! Duel 36: "Battle Beyond Hope"
- Yu-Gi-Oh! Duelist Duel 15: "Messenger From Hell"
- Yu-Gi-Oh! Duelist Duel 14: "The Thief"
- Yu-Gi-Oh! Duelist Duel 17: "The Legendary Dragon"
- Yu-Gi-Oh! Duelist Duel 49: "The First Stages of Fear!"
- Yu-Gi-Oh! Duelist Duel 46: "No Mercy"
- Yu-Gi-Oh! Duelist Duel 52: "The Promise"
- Yu-Gi-Oh! Duelist Duel 53
- Yu-Gi-Oh! Duelist Duel 54
- Yu-Gi-Oh! Duelist Duel 60: "Because We're Friends"
- Yu-Gi-Oh! Duelist Duel 73: "The Tragic Tale of the Millennium Eye"
- Yu-Gi-Oh! Duelist Duel 74: "The Precious Piece"
- Yu-Gi-Oh! Duelist Duel 91: "The Gathering"
- Yu-Gi-Oh! Duelist Duel 100: "Behold the God!"
- Yu-Gi-Oh! Duelist Duel 114: "Combo of Despair!!"
- Yu-Gi-Oh! Duelist Duel 115: "The Combo of God!!"
- Yu-Gi-Oh! Duelist Duel 116: "A Duelist's Power"
- Yu-Gi-Oh! Duelist Duel 117: "God Falls!?"
- Yu-Gi-Oh! Duelist Duel 118: "Attack that Opponent!"
- Yu-Gi-Oh! Duelist Duel 125
- Yu-Gi-Oh! Duelist Duel 131: "Where Jonouchi Waits"
- Yu-Gi-Oh! Duelist Duel 132: "The Place of Destiny"
- Yu-Gi-Oh! Duelist Duel 133: "The Unwanted Duel!!"
- Yu-Gi-Oh! Duelist Duel 138: "One Card of Life and Death!"
- Yu-Gi-Oh! Duelist Duel 140: "The Treasure"
- Yu-Gi-Oh! Duelist Duel 142: "Warriors Assemble"
- Yu-Gi-Oh! Duelist Duel 143
- Yu-Gi-Oh! Duelist Duel 163: "The Rise of Ra!"