Kou Yamori is a 14-year-old junior high school student whose dissatisfaction with daily life leads him to skip school and wander at night. He possesses a slender build, short spiky black hair, and eyes that appear black or amber, often shadowed by noticeable eye bags. His typical outfit features black baggy shorts, a white t-shirt layered under a black jacket with white stripes on the sleeves, and white leather shoes. Later, he adds an earring to his left ear and replaces his damaged jacket with a similar one. His school uniform consists of a grey suit, white shirt, striped tie, grey pants, and black belt. Despite cold weather, he frequently wears shorts, explaining he uses thermal layers underneath. In subsequent appearances, his style shifts to mirror his employer's, adopting a trench coat, black turtleneck, heeled boots, and longer hair tied in a ponytail. Initially introverted and gloomy, Kou favored solitary activities like watching ants over social interaction. During middle school, he crafted a charismatic, high-achieving persona but felt disconnected from others and devoid of purpose. Night walks offered freedom, and after encountering the vampire Nazuna Nanakusa, he resolved to become a vampire himself, seeking to eternalize this lifestyle. This newfound sense of purpose and desire to escape human constraints fuel his motivation. Kou exhibits a strong, if unconventional, sense of justice; he does not universally condemn killing. He struggles to comprehend emotions, particularly love, initially mistaking lust for romantic feelings after his first kiss with Nazuna. Though claiming indifference to loneliness, he yearns for connection, evidenced by childhood attempts to communicate via walkie-talkie. Over time, he becomes more open, assertive, and emotionally aware, yet remains prone to jealousy and possessiveness, especially concerning Nazuna. He displays covert perverted tendencies, like staring at women's breasts, and oscillates between intelligence and obliviousness, such as overlooking imminent vampire-related dangers. Repeated blood-sucking by Nazuna triggers a half-vampire state, initially causing aggressive, bloodthirsty behavior. This transformation grants superhuman abilities like enhanced strength and remote limb control, though he reverts to human form afterward. He manages transformations using a self-piercing tool. His half-vampire status defies established rules, prompting him and allies to investigate its nature. Key relationships define his journey: Nazuna Nanakusa serves as his savior and primary bond. Initially embarrassed by her lewd humor, Kou grows to prefer her authenticity over other vampires, becoming protective and jealous of her interactions, eventually acknowledging partial romantic feelings despite ongoing confusion about love. His mother, Kei Yamori, is a single parent working in nightlife who cares for him but adopts a hands-off approach, feeling inadequate due to limited time together; his father is absent after infidelity. Allies include childhood friend Akira Asai; Mahiru Seki, a former friend corrupted by vampire Kiku Hoshimi, leading to conflict and eventual reconciliation before Mahiru's death; and Anko Uguisu, initially a vampire-hunting detective who later employs Kou after he prevents her suicide. Beyond Nazuna, he befriends vampires like Seri Kikyo, Nico Hirata, and Kabura Honda (Nazuna's adoptive mother), often becoming a unifying figure among them. His story arcs involve confronting Uguisu's anti-vampire campaign, stopping Kiku Hoshimi's manipulation of Mahiru, and joining a school trip to Hokkaido where he nearly dies before reconciling with Mahiru. After Mahiru's death, Kou briefly returns to school but ultimately resumes night activities, continuing his quest to understand love and complete his vampirization. For spin-offs or other official media beyond the main series, no specific information is available.