Ko Yamori is the main protagonist of the series. He is a fourteen-year-old middle school student living in the city of Komori. Initially a model student who was well-liked by his peers and performed well academically, Ko grows weary of the structured expectations of his conventional daytime life. This dissatisfaction leads to insomnia, and he begins skipping school to wander the city streets at night, seeking an escape from his ennui.

Ko's personality is defined by a blend of friendliness and profound melancholy. He is introspective and struggles with a sense of emptiness, feeling disconnected from the goals and social rituals of his peers. He expresses a strong desire for authenticity and a life less ordinary. Despite his friendly demeanor, he can be awkward, especially in his interactions with girls, and he initially finds it difficult to grasp the complexities of emotional connection and love. As the story progresses and he becomes entangled in the world of vampires, his personality undergoes a noticeable shift; he becomes more assertive, prone to anger, and intimidating, though his underlying kindheartedness never fully disappears.

Ko's primary motivation is a deep-seated desire for transformation. This manifests as an earnest wish to become a vampire. He believes that becoming a vampire will free him from the mundane burdens of his human life and grant him a sense of belonging and excitement. His journey begins when he meets Nazuna Nanakusa, a vampire who finds his blood exceptionally delicious. Believing that he must fall in love with her to be turned into a vampire, Ko dedicates himself to spending his nights with her.

As the central figure in the story, Ko's role is to explore the nocturnal world and challenge the nature of vampires and humans. His relationship with Nazuna is the emotional core of his journey. She acts as a catalyst for his descent into the night, and their interactions explore themes of companionship, desire, and the meaning of love. Other key relationships include his friendship with the vampire detective Akira Asai, who provides guidance, and his complex dynamic with his former friend Mahiru, which forces him to confront the darkness within the vampire society he wishes to join.

Ko's character undergoes significant development as he navigates his dual existence. He learns to better understand his own emotions and those of others, gradually moving from a passive seeker of escape to an active participant in his own destiny. He must reconcile his romanticized fantasy of vampirism with the harsh and often lonely realities of the immortal condition, including the predatory instincts and societal rules that govern it.

In terms of abilities, Ko begins as a perfectly ordinary human. However, a crucial development occurs when he experiences blood loss from a vampire bite. This awakens a half-vampire state within him. In this form, Ko gains immense physical strength, superhuman speed, heightened senses, and the ability to manipulate shadows. This transformation is initially triggered involuntarily by blood loss or intense emotional distress. Over time, Ko learns to consciously activate this power, such as by using an ear-piercing kit to cause a small amount of controlled injury, allowing him to tap into his abilities for the sake of defense and control over his new nocturnal life.