Kouta is the main male protagonist of the series. He is originally from Hokkaido and later moves to Kanagawa Prefecture to attend university, where he lives in an old inn that his cousin Yuka’s family rents to him. As a child, he experienced a traumatic event that he later represses: while on a trip with his family, he met a girl who was burying her dead puppy. He gave her a hat to cover the small horns on her head, took her to the zoo, and asked her to be his friend. He also lied about Yuka’s gender to avoid making the girl jealous. When the girl discovered the lie, she followed him onto a train. His younger sister, Kanae, was terrified of the girl and tried to protect him, and in the ensuing violence the girl killed both his sister and his father. Kouta himself was hospitalized in a psychiatric ward for over a year due to the psychological trauma.

Years later, Kouta has suppressed these memories and is a kind, gentle, and empathetic young man. He is particularly protective of girls in trouble, partly because they remind him of his lost sister. He is conflict-averse and often tries to create peace around him. When he encounters a wounded and childlike girl with no memories, whom he names Nyu, he takes her in without hesitation, not realizing that she is the same person who destroyed his family. Nyu’s presence, along with the appearance of other characters such as Mayu and Nana, gradually forms an unconventional family at the inn. Kouta’s main motivation is to protect this new family and to understand his own past. His role in the story is to serve as the emotional anchor for the people around him, offering safety and kindness in a world marked by violence and prejudice.

Key relationships shape his growth. His bond with Nyu is both tender and fraught with hidden pain; he cares for her innocence without knowing her true identity. When repressed memories finally surface, he is forced to confront the fact that Nyu and the murderous Diclonius Lucy share the same body. In the anime adaptation, Kouta is able to accept that Lucy became violent as a result of the cruelty she suffered, and he tells her that he still loves her while acknowledging he cannot fully forgive her for killing his family. His relationship with his cousin Yuka is a source of steady affection and eventually, in the manga, blossoms into marriage and fatherhood. He also becomes a surrogate father to Mayu and a protector to Nana, giving them a home and a sense of belonging.

Kouta does not possess any supernatural abilities. His strengths are entirely human: his compassion, his willingness to trust others, and his capacity to endure and process trauma. His development is defined by the slow recovery of his memories and the painful reconciliation of his love for Lucy with the horror of what she did. He grows from a haunted, forgetful student into a resilient adult who can face the truth and still choose kindness. By the end of the story, he has built a life that honors the past while looking forward, having found both peace and the ability to forgive.