Kotaro Mochizuki is introduced as the younger brother of Jiro Mochizuki, and the pair claim to be the last remaining vampires of a special bloodline. They share no physical resemblance, as Jiro is a tall, dark-haired man while Kotaro is a small, blonde-haired boy with blue eyes. He typically wears Victorian-inspired attire, including a white collared shirt, an ascot, and a brown sleeveless jacket, and carries a pair of goggles around his neck. Kotaro is, in truth, the reincarnation of Alice Eve, the progenitor of the Wise Eve bloodline and Jiro's former lover, who perished in the Kowloon Shock a decade earlier. Jiro carried her blood and memories to preserve them, a burden that dictates his destiny to eventually pass them back to Kotaro and fade from existence.

Kotaro possesses a laid-back, sociable, and surprisingly cheerful demeanor given his circumstances, though he carries a considerable weight of worry beneath the surface. He displays a remarkable resilience, possessing an extremely high pain tolerance and the ability to withstand violent impacts without apparent injury. Unlike most vampires, including his brother, he is not harmed by sunlight or running water, and he initially lacks the standard vampiric abilities such as telekinesis.

Kotaro seeks to understand the world and his own future. His primary motivation is closely tied to his bond with his brother, though his role in the story is largely as a passive catalyst whose existence drives the actions of those around him. Jiro's primary goal is to give Kotaro a normal, worldly experience so he can build human friendships and stand on his own once Jiro is gone. Kotaro's abduction by the surviving Kowloon Children serves as the inciting incident that throws the main characters into the conflict of the Special Zone. Throughout the series, his identity as a dormant source blood gradually shifts him from a protected figure into the central key to the world's fate.

His relationship with his brother Jiro is the emotional foundation of the story. Jiro is both his guardian and the keeper of his past life's memories, and Kotaro's strong resemblance to Alice Eve gives their bond a complex, poignant dimension. He also forms a significant connection with the human mediator Mimiko Katsuragi, who represents the human ties Jiro hopes Kotaro will cultivate.

Initially presented as a young boy with no special vampiric powers, Kotaro is gradually developed into the reborn progenitor of a powerful bloodline. His immense vampiric powers are described as being in an unstable, dormant state, sealed away while Jiro still carries Alice's essence. The climax of his personal arc involves his maturation into an awakened source blood, a process that requires a deep, prolonged sleep following the final battle. As the reincarnation of Eve the Wise, Kotaro is a source blood, placing him at the apex of vampire potential. He does not share typical vampiric weaknesses and possesses superhuman endurance. His true abilities remain dormant for most of the series, awaiting the moment his brother returns Alice's blood and memories to him.