Tarō Kusakabe is a high school student chosen as the Mouse Knight, one of the Beast Knights defending Earth from the mage Animus and his golems. He wields the psychic power to generate a Holding Field, a protective barrier, aided by his familiar, Lance Lumiere.

Tarō shares a deep bond with childhood friend Hanako Sorano, the Mantis Knight, harboring romantic feelings for her despite her reserved nature. Fearing for Hanako's safety after her own wish increased her mortality risk, Tarō made a wish through Lance Lumiere: to heal Hanako if she sustained lethal damage while conscious, excluding instant death.

During the battle against the ninth knight-like golem, which separated the Beast Knights, Tarō saw the golem target Hanako. He shielded her, and the golem impaled them both simultaneously. With his final breath, Tarō confessed his love; Hanako acknowledged she always knew. As they fell, he activated his Holding Field to repel the golem. Tarō succumbed to his wounds, triggering his wish posthumously to heal Hanako. However, his body had already absorbed her fatal blow, making the wish mechanically unnecessary for her survival.

His death deeply affected the remaining Beast Knights. They individually honored him by wearing black during their next major battle against the golem Boedromion, treating the event as his symbolic funeral. Lance Lumiere vanished after Tarō's death.

Tarō evolved from a kind but unassuming student to one capable of decisive, selfless action driven by devotion to Hanako. He aspired to become a chef after high school.