Koichi Sakaya is the husband of Yukiko Sakaya and the father of Rio and Yuki Sakaya. He is introduced as a seemingly typical husband and father, but his behavior reveals a deep emotional and physical detachment from his wife. He sleeps in a separate bed and fails to satisfy Yukiko's needs, leaving her feeling lonely and unfulfilled. This neglect drives Yukiko to seek satisfaction on her own and eventually sets the stage for the family's unraveling. Koichi’s personality is marked by self-interest and a lack of commitment to his marriage; he abandons his wife for a younger woman, showing that he prioritizes his own desires over his family’s stability. His motivations appear rooted in personal gratification and an escape from the obligations of his marital role. In the story, Koichi’s actions serve as a catalyst: his rejection of Yukiko leads her to become involved with his own father, Gonzo Sakaya, triggering a series of illicit relationships within the household. His key relationships are defined by distance—he is estranged from Yukiko, his departure deeply affects both her and his daughter Rio, and his interaction with his son Yuki is minimal. Koichi undergoes little personal development; instead, his departure marks a turning point that forces the other characters to confront their own desires and dysfunctions. He has no notable special abilities, and his significance lies in how his neglect and infidelity set the family drama in motion.