Kou Yamori is a 14-year-old human whose dissatisfaction with daily life drives him to wander nights. Initially gloomy and reserved, he prefers solitary activities like observing ants over socializing. During middle school, he crafts a charismatic, upbeat facade as an honor student, masking his inability to form genuine connections and confusion over emotions like love. His aimlessness shifts upon encountering vampire Nazuna Nanakusa, who introduces him to the nocturnal world. This sparks his new purpose: to become a vampire and live eternally in night's freedom. Achieving this requires falling in love with Nazuna—a struggle due to his emotional ambiguity. Physically, Kou sports a slender frame, short black spiky hair, and eyes appearing black or amber. He typically wears a white t-shirt under a black jacket with white-striped sleeves, black shorts, and white shoes. After damaging his jacket, he replaces it with a similar one. Later, he acquires a left earring to control emerging half-vampire abilities. As he ages, he grows taller, ties his hair in a ponytail, and adopts a trench coat, black turtleneck, and boots resembling his employer Anko Uguisu. Kou's personality blends a strong sense of justice with unconventional morals; he loudly judges perceived immorality yet doesn't universally condemn acts like killing. Compassionate, he often sees good in adversaries, leaving him vulnerable to deception. Despite claiming indifference to loneliness, childhood attempts to connect via walkie-talkie reveal his yearning for bonds. He exhibits lecherous tendencies—staring at breasts or downloading explicit images—though embarrassment follows these impulses. Intelligent, he detects lies and engages complex conversations, yet remains oblivious to immediate dangers. Nightly explorations gradually build his confidence and emotional openness. After preventing Anko Uguisu's suicide, he manifests half-vampire traits—a hybrid state granting enhanced strength, speed, and durability. This transformation fuels a more assertive, anger-prone personality with occasional bloodlust. During one confrontation, rage nearly drives him to kill friend Mahiru Seki. They later reconcile amid a conflict involving vampire Kiku Hoshimi in Hokkaido. Following Mahiru's death and other losses, Kou temporarily distracts himself by returning to school before resuming his nocturnal life. His incomplete half-vampirism carries a one-year deadline for full transformation or death. Key relationships define his journey. With Nazuna, he remains confused between lust and love; her kisses excite him, yet romantic feelings stay uncertain. He grows fiercely protective and jealous, attacking Haruka Nanakusa for expressing interest in her. His friendships include supportive classmate Akira Asai and Mahiru Seki, whose descent into vampirism under Kiku Hoshimi fractures their bond. Initially enemies, he and employer Anko Uguisu develop mutual respect. He also allies with vampires Seri Kikiyou, Kabura Honda, and Hatsuka Suzushiro despite early conflicts. Raised by single mother Kei Yamori after his father's infidelity caused their divorce, Kou expresses gratitude toward her for enabling his nocturnal encounters, reflecting a complex but appreciative bond.