Taito Kurogane began as a seemingly ordinary Miyasaka High freshman, his self-perception shaped by an abandoned elementary school karate career cut short by a snapped tendon leg injury that cemented his view of himself as average.

Nine years before the main events, his path crossed with Saitohimea, a Vampire designated a "Most Ancient Sorcerer." She formed a contract by injecting him with an immortality-granting poison. This conditional immortality allows resurrection after death, unless he suffers seven deaths within a fifteen-minute window, leading to permanent demise. Their meeting was violently interrupted by Kurenai Hinata, who attacked with a demon, killing Taito six times in rapid succession. To save Taito, Saitohimea agreed to leave with Hinata, resulting in the sealing of Taito's memories of both her and the contract.

His ordinary existence shattered when he sacrificed himself to save classmate Mirai Andou from a truck, an impact that decapitated him. His body spontaneously regenerated, restoring his sealed memories and revealing his immortality. This regeneration also healed his chronic leg injury, fully restoring his physical mobility. Merely remembering Saitohimea's name enabled her escape from interdimensional imprisonment, leading to their emotional reunion at their childhood meeting place. Taito pledged to protect her from entities known as Tenma—translated as demons or evil spirits, but depicted as distinct, godlike Eldritch Abominations.

Following the reunion, he joined the Miyasaka High student council alongside Saitohimea, collaborating with its president, Gekkou Kurenai (Hinata's twin brother), and Mirai, Gekkou's lightning-demon familiar. His relationship with Gekkou evolved into a competitive, brotherly dynamic, marked by mutual challenges like endurance contests. Concurrently, he navigated complex interpersonal ties, including his deep romantic bond with Saitohimea and unresolved feelings involving childhood friend Haruka Shigure, who harbored affection for him.

His abilities expanded beyond conditional immortality. He gained the power to manifest deadly blue flames around his arm, though each activation consumes one of his lives due to its lethal cost. He later acquired "Dark Rabbit Mode," enabling him to reject and erase existences by downloading layered prophecies into his mind. However, this overloads his brain, causing meltdowns that also cost him lives. Additionally, he formed a contract with the feline familiar Nyankichi, summoned using his blood, which grants him spellcasting abilities including dark vision and magic-cursing seals.

His character trajectory centered on protecting Saitohimea from threats like Hinata, who sought to revive the sealed Vampire Bahlskra, and grappling with a world-spanning prophecy foretelling catastrophic events.