Shuichi Kagaya is a high school student seeking normalcy yet haunted by his monstrous abilities. He transforms into a hollow, dog-like mascot costume armed with a functional revolver and featuring a zipper along its spine. This zipper allows others to enter his body, enhancing his physical capabilities and enabling shared control. His monster form grants superhuman strength, agility, endurance, and an acute sense of smell that remains somewhat present even in his human state. Injuries sustained while transformed can prove fatal if he reverts before healing. Significant gaps mar his memories. He once belonged to a close-knit childhood group with Elena Aoki and Kaito. Elena, his ex-girlfriend, erased his memories using her abilities after a traumatic event involving a creature impersonating their deceased friend Honoka. This entity erased his parents when they failed to recall Honoka, prompting Elena's memory alteration to protect Shuichi. Consequently, Shuichi initially recalls neither his past relationship with Elena nor her role in granting him his transformation ability via an alien entity. Initially timid, academically gifted, and averse to conflict, Shuichi is forced into Clair Aoki's quest for vengeance against Elena—who murdered their parents—after rescuing Clair from a fire. Clair's ruthlessness terrifies him, contrasting his lingering humanity. Facing repeated threats and uncovering his past, Shuichi becomes progressively colder, more violent, and willing to sacrifice his well-being. His deepening codependency with Clair mirrors his monster form's symbolic emptiness and need for others to "fill" him. This evolution culminates in acts like killing a vengeful gang member to protect Clair. His abilities evolve through partnerships. When Clair enters his form, their synchronized goals amplify his strength and combat effectiveness. Later, under life-threatening conditions, he discovers a fusion ability: if he and his host share identical objectives and the host nears death, they merge into a single, more powerful entity. This fusion combines their memories, physical traits, and weaponry—Clair's fusion yields a katana, while another partner results in dual pistols. This fused state temporarily overcomes Kaito's regeneration. Key relationships define his journey. His bond with Clair shifts from manipulation to mutual reliance, with her vowing suicide if he dies. Elena's obsession drives much conflict; she violently opposes anyone else entering his form. His connections to former friends like Kaito deteriorate as Kaito descends into madness, leading to unresolved confrontations. Physically, Shuichi has brown eyes and black-brown hair. He initially wears glasses and bears a neck scar from Elena. His monster form features light brown fur with darker accents on the head.