Kōshirō Saeki is a twenty-eight-year-old man of large build who typically appears with an unshaven five‑o’clock shadow and works as a wedding planner for a marriage‑arranging company. He lives with his father, Zenzo, in Tokyo. Kōshirō grew up as the child of divorced parents; after the separation he stayed with his father while his younger sister, Nanoka, lived with their mother. As a result, he lost contact with her and eventually forgot he even had a sister. His own romantic life is strained: his girlfriend of two years, Shoko, breaks up with him, claiming he is too cold and distant. This rejection leaves him feeling emotionally hollow. Kōshirō’s personality is initially withdrawn and gruff; he struggles to articulate his feelings and often comes across as boorish or detached. Underneath this exterior, however, he experiences subtle and powerful emotions but lacks the eloquence to express them. His job is ironic in that he arranges marriages while being unable to maintain his own relationship.

The story begins when Kōshirō encounters Nanoka on a train without knowing she is his sister. He is drawn to her innocent smile and later, after a chance outing to an amusement park, they discover their true relationship when they meet their father. Nanoka has moved in with them to attend high school. Kōshirō is immediately conflicted: he feels a natural protective instinct toward his sister, yet he is also attracted to her in a way that disturbs him. His internal struggle becomes the central conflict of his character. He tries to suppress his feelings by being brusque and distant with Nanoka, but his desire and affection only grow. In desperation, he moves out of the family home to remove himself from temptation, but Nanoka continues to visit him, cooking for him and spending time with him. This only intensifies his turmoil.

Kōshirō’s key relationships include his father, Zenzo, who is loving but somewhat oblivious to the siblings’ tension, and his coworker and supervisor, Kaname Chidori, who becomes suspicious of the siblings’ closeness. Kaname tries to intervene, but Kōshirō is unable to explain his situation. His relationship with Nanoka is the core of the narrative: it evolves from awkward cohabitation to a deep, forbidden romance. Kōshirō’s motivation is to do what is socially acceptable and to protect Nanoka, but his own emotional and physical needs eventually override his restraint. Over the course of the series, he develops from an emotionally closed‑off man into someone who openly acknowledges his love, despite the societal taboo. He and Nanoka share a passionate kiss and later become intimate. After grappling with guilt and contemplating suicide, they ultimately choose to live together and continue their relationship, accepting the consequences.

Notable abilities are not supernatural or physical; instead, Kōshirō’s defining trait is his capacity for intense, unexpressed feeling and his eventual willingness to confront his own emotions. He is a fundamentally decent person who is caught between his sense of responsibility and his genuine love for his sister. His journey is one of self‑discovery and acceptance, marked by quiet introspection and painful decisions.