He is generally a kind person, but when Dueling he becomes quite ruthless, showing no mercy to his opponents, and especially to those who possess a Number. When his brother Haruto fell sick, Dr. Faker told him that it was due to the Number holders and the Astral World. As a result, Kaito became a Number Hunter, accompanied by a robot called Orbital 7.
Kaito has light grey eyes, pale skin and blonde hair that is curled up in a point with a dark green spiked front. His attire includes a black trench coat with a high collar over a grey shirt, fingerless black glove on his right hand, and white pants with black boots. He also has a belt with two purple boxes for his cards.
Unlike any other regular D-Pad version of a Duel Disk, Kaito's disk is a blue and single-bladed disk that curves outside, though he uses a normal D-Pad in "Destined Meeting!!". He can Duel without a Duel Gazer; however, his left eye turns red and several marks resembling a Duel Gazer are seen when he Duels. When Kaito Duels, his black outfit switches to white in a process called "Photon Change".
After he wins a Duel with a "Number" user, Kaito uses his "Photon Hand" which enables him to extract the "Number" card from their users, though the process also steals their soul. Having lost their souls in this process, the original holders of each "Number" card ages dramatically. Kaito possesses a "Duel Anchor", which he uses to prevent a "Number" user from running away, until the Duel is over. Also, with his tattoo-like Duel Gazer, Kaito is not only able to see Augmented Reality as with any standard Duel Gazer but is also able to see Astral, who can't be seen through normal means, although not as clearly.
In the past, Kaito lived with his brother Haruto in a small, wooden villa where they would play all day. They used to hunt butterflies together, but Haruto always freed them after hunting them. Kaito used to praise him for being that gentle. One day, Mr. Heartland took Haruto away by force following Dr. Faker's orders and saying that he has a special power within him. Kaito tried to rescue Haruto, but Mr. Heartland fled by car.
Some time later, when he goes to Mr. Heartland's place, Haruto initially doesn't recognize him. Kaito hugs him and asks Mr. Heartland what he did with him, who explains that Haruto has the power to communicate with the spirits of the Astral World, and that they need him to destroy it, or our world won't be saved. Kaito tells him to return Haruto back to normal, and Mr. Heartlands says that his soul had probably already left, but that there is a way to heal him, which is to become a Number Hunter and collect all the "Numbers" - and at that time, Haruto catches a butterfly with his hands and crushes it. Shocked, Kaito accepts Mr. Heartland's order. From that day on, using Heartland Tower as a base, Kaito found and defeated the holders of the "Numbers", under Dr. Faker's command.
When a bandit attempted to rob a shopping mall, Kaito appeared with Orbital 7, using the latter's powers to create a barrier where only Number holders can move. He quickly defeated the bandit and obtained his "Number 20: Giga-Brilliant", departing afterwards.
He later found Yuma. Orbital 7 freezes the time right when Yuma would be ran over by a car, saving him, and seeing he has "Number" cards as well, forced him into a Duel. Kaito totally overpowered Yuma during the Duel, Summoning two "Numbers" during his fist turn, and mostly rendering his efforts useless with his signature card, "Galaxy-Eyes Photon Dragon", but at the instant when he would win, Orbital 7 told Kaito that his brother Haruto is badly injured, making Kaito cancel the Duel and flee.
Afterwards, when Yuma and his friends were escaping the Heartland Theme Park, Kaito and Orbital 7 went to meet them. Kaito challenged Yuma to a Duel in order to obtain his "Numbers", but Mr. Heartland showed up from the upper stairs, interrupting them. Mr. Heartland smiled and ordered Kaito to defeat the messenger from the Astral World in front of him. However, Kaito said that he didn't become a Numbers Hunter because of his orders, and triggered Orbital 7's self-explode mechanism, kicking him towards Yuma's gang, which destroyed part of the bridge. This made Yuma and his friends fall in the water under them, which allowed them to escape. Mr. Heartland told Kaito that this action was a big act of disobedience towards him and Dr. Faker. Kaito and Mr. Heartland, both irritated, stared at each other for few seconds. Kaito then turned around and left the room, ordering Orbital 7 to go with him.
Cologne explained that Kaito has became a Numbers Hunter to protect his little brother, who is ill. Astral told Yuma that even though Kaito's reasons were noble, as long as he continues being a Numbers Hunter, someday they'll have to fight each other. Yuma said that he knows that, and when that happened, he will not lose.
Upon locating another Number Kaito arrived on the scene, only to find it had already been claimed by Shark, who now acted as a Number Hunter himself. When Luna lamented his situation, which forced him to act as a puppet for Haruto's sake, Kaito was infuriated and tried to use Photon Hand on them all, but Luna used a charm to protect herself, Shark, Yuma, and Astral, then teleport them away from Kaito.
Due to his disobedience at the Heartland Theme Park, Mr. Heartland hired Kyoji Yagumo to hunt Numbers in his place.  Days later, Shark and Luna noticed that he also was watching Yuma's battle against Eviluder. The appearance of "Number 42: Starship Galaxy Tomahawk surprised all of the Number Hunters present. Kaito was startled as well by Kyoji's appearance after the Duel, and witnessed his replacement declare the beginning of "Numbers War". Declaring he was sick of it as soon as Kyoji left, Kaito departed.
That night, Kaito picked up Haruto and left Heartland Tower, blowing it up in the process. He told Haruto that Kaito was taking him somewhere safe so he wouldn't be caught up in the chaos of the war, but Heartland Tower was also not a safe place for them anymore. Unfortunately, they were ensnared by spider webs strung between buildings, and Haruto was taken by Kyoji, the one who set the trap. While Kyoji declared he knew Kaito would do such a thing, Kaito began a duel against Hishakaku, Kyoji's second assassin. The effects of "Line World" and "Number 72: Line Monster Chariot Hisha" kept Kaito in a tight spot, leaving "Galaxy-Eyes Photon Dragon" in Hishakaku's Graveyard.
Then Yuma, who noticed the light of "Galaxy-Eyes" when it was summoned, appeared on the top of the building Kyoji is standing on, and ordered him to release Haruto. Kyoji pushed Haruto off the building in response, keeping him up by a strand of spider thread. Kaito lost his composure at this point, which Hishakaku believed he could use to his advantage. "Galaxy-Eyes" was then taken away by Hishakaku, who promptly used it against Kaito to inflict massive damage. Haruto began to respond to Kaito's pain by activating his powers, which prompted Kyoji to escape. Orbital 7 and Yuma tried to pursue him, but end up inadvertently sabotaging each other's attempts. Furious, Kaito insisted he didn't need Yuma's pity and vowed to crush Hishakaku.
In a seemingly last-ditch move, Kaito sent his entire Deck to the Graveyard through "Card of Desperation" and attempted to One Turn Kill Hishakaku through a combination of "Galaxy Dragun" and "Dragon Buster", but Hishakaku stopped his move through "Negate Attack". Just when it seems Kaito has lost, however, he used "Graveyard Rebound" to prevent himself from losing. Declaring he would go to any lengths for Haruto's sake, he used the card to bring back his "Eternal Bond" Trap, reclaim "Galaxy-Eyes", and boost its ATK to 7100, defeating Hishakaku in a true One Turn Kill. While Hishakaku praised Kaito for his Duelling skills, he triggered some bombs attached to the web they were dueling on, causing Kaito, who had no energy left to get away, to fall towards his death. Yuma jumped after Kaito with the intention of saving him.
Orbital 7 caught Yuma and Kaito and managed to divert them into water, thus saving them, and Yuma carried Kaito back to his house to let him rest. When Kaito woke up, he deduced that he was in Yuma's house after wandering into Yuma's attic and finding a picture of his parents. Cologne started watching him, but quickly pretended she wasn't when Kaito noticed he is being watched. Then Kaito collapsed, prompting Yuma to bring him back down to bed and explain why Kaito was in his house. Kaito immediately challenged Yuma to a duel, repeating that he didn't want sympathy from his enemies, but someone's stomach growled just as they were getting ready. Kaito quickly denied it was him, while Yuma admits it, so Kotori made lunch for both of them. Yuma tried to help, but they ended up burning the eggs. Kaito then attempted to cook the eggs, but when asked about how he could cook his mind got taken off the cooking when he remembered Haruto's current situation. This caused Kaito to burn the eggs while he wasn't paying attention, much to his embarassment. Luckily, Akari and Haru brought their own ingredients and cooked everything themselves.
While eating, Kaito was questioned by how he knew Yuma, to which he began to reply that he was a natural born-Duelist, just like Yuma. Yuma spat out whatever he was eating on Kaito by mistake, but quickly insisted that they do Karaoke together and Kaito meant to say "duet". (This was because Akari did not approve of Yuma dueling.) Kaito was surprised by this, but did not object. Later, Kaito asked Yuma about his family, and asks if Yuma's father was looking for the door to the Astral World. When questioned about his own family though, Kaito declined to tell Yuma anything. Later, while piecing the events regarding Yuma's parents, Astral, and Haruto together, an eagle crashed through the window and dropped a note that stated the third assassin, Shadow, had taken Kotori hostage. Kaito instead Dueled him and won, but the assassin vanished, leaving only "Number 13: Cain's Doom" in his wake. As soon as he won, Kaito left with Orbital 7, declaring they were back to being enemies, and retreated to his new base. 
Some time later, he dueled Shark for his Numbers. Shark used "Anchor Whale" to nullify "Galaxy-Eyes". Shark continued to gain an edge through destroying "Galaxy-Eyes" and a combo of his "Number 47: Nightmare Shark" and "Bahamut Shark", but Kaito used "Hyper Galaxy" to bring back his ace and reduce Shark's Life Points to 1600. Unfortunately, as Shark held "Number 31: Abel's Doom", and Kaito still had "Cain's Doom", the trap Shadow had set was complete, causing the two "Numbers" to Summon themselves, reduce both their life points to 1000, and wipe out both players' fields, setting the stage for a double knockout. Yuma began a Duel to save them, paying 2002 of his Life Points to offer half the amount to each of them as "Cain's Doom" and "Abel's Doom" continued to attack each other. Luckily, Yuma managed to win through "Number S39: Utopia One, rescuing the two from a narrow defeat.
Later, at night, Tetsuo searches for Yuma, as he forgot his pendant, which catches the attention of a nearby Kaito. When Yuma and Kotori find them, Tetsuo is down on the ground, soulless, and Kaito is standing in front of him. In order to retrieve Tetsuo's soul, Yuma is forced to Duel Kaito.
Kaito quickly Summons a high-Level monster and makes Yuma defenseless, but The Door of Yuma's dreams appeared before Yuma and told Yuma to open it and he will gain new strength in return. Yuma does so and Astral appears before him and tells Yuma to activate "Monster Reborn" to bring back "Ganbara Knight", then Overlay the two Level 4 monsters to Xyz Summon "Number 39: Utopia". When "Utopia" appears, it causes Kaito's D-Pad to malfunction, forcing him to retreat and give Tetsuo's soul back.
Prior to his Duel with Yuma, Kaito owned "Number 10: Illumiknight" and proceeded to win "Number 20: Giga-Brilliant" from a criminal. He went on to win "Number 72: Line Monster Chariot Hisha" from Hishakaku and "|Number 13: Cain's Doom" from Shadow. The latter card was eventually passed to Astral.
Kaito plays a "Photon" Deck, focused on swarming the field with LIGHT-Attribute monsters, which he can then use to Xyz Summon his "Number" monsters or Summon his ace card, "Galaxy-Eyes Photon Dragon". Kaito also uses several cards to counter his opponent if they steal his "Galaxy-Eyes Photon Dragon" such as "Galaxy Dragun" and "Dragon Buster".
|Unknown Duelist||SP1||Win (off-screen)|
|Tetsuo Takeda||SP1||Win (off-screen)|
|Yuma Tsukumo||SP1||No result|
|Number 20's holder||7||Win (off-screen)|
|Yuma Tsukumo||8-9||No result|
|Ryoga Kamishiro||29-32||No result|
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