Kō Yamori is a 14-year-old junior high student plagued by dissatisfaction and insomnia, leading him to wander at night. He charms classmates with a popular facade but feels profoundly disconnected, lacking purpose or understanding of love. His world shifts upon meeting Nazuna Nanakusa, a vampire who reveals nightlife's freedoms. Seeing vampirism as liberation from human constraints, he resolves to become one, requiring him to fall in love with Nazuna—a struggle complicated by his confusion between lust and deeper feelings. Physically slender with short spiky black hair and occasionally amber eyes, his standard attire features black shorts, a white t-shirt under a black jacket boasting white-striped sleeves, and white shoes. He later acquires a silver earring and replaces a damaged jacket with a similar one. During a frigid Hokkaido school trip, he stubbornly wears casual clothes, claiming thermal layers suffice. After working for Anko Uguisu, his style evolves: a trench coat, black turtleneck, heeled boots, and his grown-out hair tied in a ponytail—potentially honoring his deceased friend Mahiru Seki. His personality blends gloominess, introspection, and an unorthodox moral compass that doesn't always condemn killing. He exhibits lecherous tendencies despite embarrassment over lewd topics. Though initially anxious about rules, he adapts to nocturnal freedom through Nazuna. Key traits include compassion, trust in others, and naivete, leaving him open to deception. After becoming a half-vampire—triggered by extreme stress or blood loss—he gains supernatural abilities but battles violent bloodthirst and heightened aggression, even in human form. Anko provides an ear-piercing kit to manage these transformations. Relationships define him. With Nazuna, he evolves from finding her "uncomfortable" to valuing her deeply, though unsure if it's love. He protects her fiercely and prioritizes her safety. His bond with childhood friend Akira Asai endures his school truancy, while his friendship with Mahiru fractures over Mahiru's involvement with vampire Kiku Hoshimi. After reconciling during a confrontation with Kiku in Hokkaido, Mahiru dies, deeply affecting Kō. He later allies with former adversaries like employer Anko, and vampires Seri Kikiyou and Kabura Honda. His journey involves investigating his half-vampire state with Nazuna and Anko, clashing with Kiku's faction, and enduring Mahiru's loss. Despite grief, he resumes night walks with Nazuna, underscoring their bond. Ongoing struggles include controlling his hybrid nature, comprehending love, and balancing loyalties between human and vampire worlds.