Kou was once a top-performing, well-behaved middle school student, but a creeping sense of emptiness and lack of fulfillment caused him to lose interest in his everyday life. Disillusioned with the expectations placed on him and unable to sleep at night, he began sneaking out to roam the city under the cover of darkness. His nocturnal solitude was interrupted when he met Nazuna Nanakusa, an eccentric and free-spirited vampire.

Upon learning that falling in love with a vampire is a prerequisite to being turned, Kou becomes determined to fall in love with Nazuna and embrace the night as his true calling. Along the way, Kou builds connections with various other vampires and humans alike, gaining friendships he never had in his previous school life and beginning to understand what he truly wants out of life and unlife.

Kou is a teenage boy with short, dark hair and sharp golden-yellow eyes that glow subtly in the dark. His attire is casual and somewhat disheveled, often consisting of a dark hoodie, shorts, and sneakers, fitting his low-maintenance and nocturnal lifestyle.

As the story develops, subtle changes in his appearance reflect his internal transformation, especially after his partial vampire awakening, during which his eyes and aura become more intense. His expressions shift from blank and bored to alert and emotionally invested as he opens up to others.

Initially, Kou is quiet, detached, and emotionally withdrawn, having become numb to the routine of daily life. He displays a maturity beyond his years but also harbors a deep sense of confusion and loneliness. His encounters with Nazuna and the night world awaken a curiosity and excitement in him that had long been absent.

As the series progresses, Kou becomes more compassionate, courageous, and introspective. He learns to value friendship and begins standing up for others, even risking his life to protect those he cares about. Despite his awkwardness and occasional overthinking, Kou is kind-hearted and willing to challenge himself to understand the complex emotions of both humans and vampires.

• Discovery of the Night
  Kou’s story begins when he leaves his house at night to escape the pressures of his daily life. He meets Nazuna Nanakusa, who introduces him to the strange freedom of the nighttime. Fascinated by her and the allure of the vampire lifestyle, Kou decides that he wants to become a vampire and that in order to do so, he must fall in love with her.

• Encounters with Other Vampires
  Kou’s budding relationship with Nazuna draws the attention of a group of vampires led by Nico Hirata, who inform him that he must become a vampire within a year or he will be eliminated. Rather than be intimidated, Kou chooses to befriend the vampires, winning their trust and forming bonds with each one over time.

• Clash with Anko Uguisu
  Kou’s journey takes a dark turn when he meets Anko Uguisu, a vampire-hating detective. Their ideologies clash Kou believes vampires and humans can coexist, while Anko seeks to destroy all vampires. During a dramatic Halloween confrontation, Kou prevents Anko’s suicide, awakening a hybrid form within himself, suggesting he is part vampire.

• The Hybrid Mystery and Fallout
  As Kou begins to explore the nature of his half-vampire identity, he allies with Nazuna and Anko now revealed to be Kyouko Mejiro to uncover the truth behind his transformation. During this time, he has a devastating fallout with his friend Mahiru Seki, who becomes entangled with the vampire Kiku Hoshimi. This conflict eventually leads Kou to a dangerous confrontation in Hokkaido, where he nearly loses his life but manages to reconcile with Mahiru.