Kaito Tenjo, known in the Japanese version as Kaito Tenjo, is a central character first introduced as an antagonist and rival in the Zexal series. He is an eighteen-year-old Number Hunter, a role he reluctantly undertakes to save his younger brother, Hart Tenjo.

Kaito's background is rooted in his deep love for his brother. Living under the care of their father, Dr. Faker, Hart fell victim to a mysterious illness. Dr. Faker and his subordinate, Mr. Heartland, convinced Kaito that the only way to cure Hart was to collect all one hundred Number cards, which they claimed were the source of the sickness. To this end, Kaito was equipped with technologically advanced abilities and began his mission, accompanied by his loyal but eccentric robotic companion, Orbital 7.

In terms of personality, Kaito is generally a reserved and kind individual, but when he duels, he becomes ruthlessly cold and focused. He often announces his arrival with a distinctive whistle and declares his intent to collect his opponent's soul along with their Numbers. This menacing demeanor is a direct result of the emotional manipulation he suffered, as he genuinely believed that Number holders were evil and that their suffering was necessary to save his brother. Despite his harsh exterior, he despises cruelty for its own sake and views his hunting as a grim duty rather than a pleasure. His primary motivation is Hart, whom he considers his everything, and he constantly pushes his own body past its limits using a process called Photon Transformation, ignoring the damage it causes.

Kaito's role in the story evolves significantly. Initially a formidable adversary to the protagonist Yuma Tsukumo and his ethereal partner Astral, Kaito is driven by his goal to collect the Numbers they possess. Over time, he discovers that Dr. Faker and Mr. Heartland lied to him, that the Number holders were innocent, and that his actions caused immense harm. This revelation transforms him from a villain into a crucial ally. He becomes a key fighter against greater threats, including the Arclight family and later the Barians from Barian World.

Key relationships define his journey. His bond with his brother Hart is the core of his character, providing both his initial tragic flaw and his eventual redemption. His relationship with Yuma Tsukumo shifts from rivalry to deep respect and friendship, as Yuma's unwavering belief in others helps Kaito learn to trust again. He also develops a mutual respect with Ryoga Shark Kamishiro, and his path becomes intertwined with Mizael, a Barian who also wields a Galaxy-Eyes dragon, leading to a fateful final confrontation.

Throughout the series, Kaito shows considerable development. He begins as a lonely, deceived pawn, but by the end of Zexal II, he has become a hero who willingly fights for others, having made peace with his past sins and his role as a duelist. He comes to see dueling not just as a means of hunting, but as a way to connect with others and fight for the future.

Kaito possesses several notable abilities. As a Number Hunter, he can use the Photon Hand to physically extract Number cards from a defeated opponent, a process that also steals the victim's soul and rapidly ages them. He also wields a Duel Anchor to prevent prey from escaping. These are technological gifts from Dr. Faker. When he duels, his left eye turns red, and his standard black coat transforms into a white coat in a process known as Photon Transformation, which enhances his power but strains his body. His deck is a Photon and Galaxy-themed deck, and his ace monster is the mighty Galaxy-Eyes Photon Dragon. After receiving some of Hart's special powers, he can also see Astral and gain the strength to summon even more powerful forms, such as Neo Galaxy-Eyes Photon Dragon.