Yuuhi Amamiya, a college student, carries profound childhood trauma. His detective father was murdered by a traitorous colleague, and his mother abandoned him during the funeral. Taken in by his grandfather, Yuuhi endured physical and psychological abuse designed to instill deep distrust. Beatings for attempting friendships and confinement in chains within a basement shaped his apathetic, misanthropic worldview. He became emotionally dependent on these chains as symbols of isolation until meeting Samidare Asahina.

Initially cynical and withdrawn, Yuuhi exhibited little concern for the world's fate when recruited as the Lizard Knight to combat an apocalyptic threat. Meeting Samidare, the "princess" of the Beast Knights, shifted his motivations after she revealed her plan to save the world only to destroy it. Yuuhi transferred his psychological chains to Samidare, swearing absolute loyalty to her goal. He declared he would commit murder or die for her happiness, viewing her as his sole purpose.

His abilities centered on Domain Control, a telekinesis granted via a magic ring. Initially weak, allowing brief levitation or enhanced jumps—dismissively termed his "Retreat Gimmick"—training unlocked advanced techniques: the martial arts spear technique **Houtengeki**, inherited from fellow knight Shinonome Hangetsu; **Babylon**, a dome of weak domains boosting his speed tenfold while slowing opponents for tactical advantage; and **Negai Kanau Hikari**, a combined attack with other Beast Knights generating a city-level energy blast. Yuuhi adapted Hangetsu's martial prowess to compensate for his frail physique. His strategic intellect was notable, defeating early enemies through deception and later concealing true capabilities by studying allies' weaknesses during battles.

Yuuhi's relationships evolved significantly. His loyalty to **Samidare Asahina** stemmed from romantic devotion; he prioritized her ambitions above all, though he ultimately confronted her to prevent global destruction. His lizard familiar, **Noi Crezant**, frequently challenged his nihilism and Samidare's manipulation. **Shinonome Hangetsu**, initially a rival, became a respected mentor; Yuuhi wept openly at Hangetsu's death, demonstrating newfound emotional capacity. Interactions with the **Beast Knights** began with cold detachment, but bonds formed through shared battles softened his cynicism, leading him to later risk his life protecting them.

Yuuhi's character arc transitioned from nihilism to purpose-driven resolve. After Samidare defeated the primary antagonist Animus and declared her intent to destroy the world, Yuuhi fought multiple Beast Knights simultaneously to shield her plan. He ultimately defeated Samidare in combat. Driven by love and shared experiences, this act persuaded her to abandon her goal. Post-conflict, Yuuhi achieved his childhood dream of becoming a detective and maintained his relationship with Samidare. His journey concluded by overcoming inherited trauma, forging genuine connections, and actively protecting the world he once sought to annihilate.