Claire Aoki is a fair-skinned young girl with long light brown hair, brown eyes, and distinctive freckles across her face. Her figure appears more developed than that of her older sister Elena. She typically wears casual home clothing like a shirt and panties, but dons a dark blue swimsuit for practical reasons when operating inside Shuichi Kagaya's hollow monster form. Her parents were strict hypocrites—her mother had an affair with a younger coworker, while her father was a corrupt politician exploiting his position. Elena killed their parents, disguising it as death by poisonous gas, leaving Claire traumatized and disbelieved by others. This isolation drove Claire to attempt suicide via arson, until Shuichi rescued her in his monster form. Discovering his secret, she blackmailed him into helping find Elena, initially viewing him as a tool and exhibiting abusive, manipulative behavior. Her early personality was marked by ruthlessness, perverse humor, and a willingness to undress provocatively to unsettle Shuichi. Lacking innate combat abilities, Claire relies on entering Shuichi's monster form via a back zipper. Inside, she synchronizes with his thoughts and emotions, gaining precise control over his movements and accessing superhuman strength and speed. This synergy enhances their combined combat effectiveness, often wielding a katana. Later, Claire uses a golden coin to wish for enhanced abilities, gaining time-slowing perception that heightens her reflexes and speed. She eventually achieves full fusion with Shuichi, triggered by shared goals and emotional honesty, resulting in a more powerful merged form. Claire's character evolves: her initial cold pragmatism and death-seeking attitude shift as she bonds with Shuichi. She vows to die with him if he perishes first and develops genuine romantic feelings. Despite this growth, she retains a capacity for ruthless actions, killing without hesitation when necessary—such as orchestrating mass murder to escape a mountain conflict—though she later exhibits remorse. Jealousy surfaces when other girls interact with Shuichi, particularly Chihiro Yoshioka, whom she intimidates. Her backstory reveals suppressed memories manipulated by Elena, including her status as Elena's paternal half-sister from their father's affair. Her relentless pursuit of Elena stems from a complex mix of hatred, unresolved trauma, and a need for closure. Her surname "Aoki" means "blue/green tree," while her given name "Claire" translates to "crimson love." Key relationships define her journey: with Shuichi, she transitions from exploitative partnership to deep interdependence; with Elena, it remains fraught with betrayal and violence, underscored by their twisted familial ties. Claire's cunning and strategic mind often drive their decisions, compensating for her physical vulnerability outside Shuichi's form. Her development culminates in a hardened resolve to protect Shuichi and confront her past, even as unresolved conflicts with other characters like Kaito hint at ongoing struggles.