Shuusei Usui fulfills the role of a Zweilt guardian wielding Kami no Me, which sharpens his perception and fortifies defensive powers. His lifelong bond with Hotsuma Renjou anchors his story, forged in childhood through loyalty and mutual trauma. A defining moment saw Shuusei deploy fire-based abilities to stop Hotsuma’s suicide attempt—an act that scarred his chest with permanent burns and compromised his health, intensifying Hotsuma’s guilt while cementing their reliance on each other as irreplaceable pillars of stability. Emotional neglect shadows Shuusei’s past, marked by overhearing his parents voice regret over his existence, compounding his struggle with self-worth. A prior incarnation as Hotsuma’s wife adds complexity, their union once focused on bearing children to strengthen the Zweilt. This cycle of rebirth underscores enduring ties and unresolved grief spanning lifetimes. His reserved exterior masks sharp intellect and concealed anguish, often deflected through polite formality or wry humor. A self-sacrificial streak drives him to endure lethal risks for others’ sake, exemplified by shielding Hotsuma from pain at his own expense. Though outwardly composed, private battles with despair emerge when he perceives himself obsolete in Hotsuma’s life amid Yuki’s rising influence. Combat prowess centers on tactical defense: crystal spheres conjure barriers and channel energy, while twin blades, Kurai Kurou, enable close-quarters strikes. Chronic frailty restricts direct combat, emphasizing his strategic support role. Growth unfolds through tentative bonds, such as his guarded rapport with Tsukumo. Initially aloof, he eventually accepts her friendship, symbolized by sharing ice cream and promising future companionship. A critical juncture arises during captivity under Opast Ashley, where latent suicidal urges surface—halted only when Hotsuma invokes Kami no Koe to demand his survival. Post-rescue, Shuusei publicly kneels before Yuki, pledging protective service, signaling a departure from prior emotional evasion. Connections extend beyond Hotsuma: he safeguards Touko during a mission without romantic intent, and his dynamic with Yuki shifts from detached coexistence to sworn allegiance following her aid in his rescue. Physically, brown hair and golden eyes frame a slim frame, his "untouchable" elegance contrasting profound vulnerability hidden beneath.