Kōsuke Nitta is a character from the 2020 live‑action film BELA: Humanoid Monster, a spin‑off of the Humanoid Monster Bem franchise. He works at an advertising agency and is assigned to produce a feature on the classic anime Youkai Ningen Bem, which has since faded from popularity. While exploring an abandoned building with a junior colleague to search for material, Nitta stumbles upon a rumored lost final episode of the series. The shocking content of that episode unhinges him, triggering a gradual descent into obsession and irrational behavior.

Nitta’s personality initially appears ordinary—a salaried employee with a wife, Ayumi, and a young son, Hinata. However, after his discovery, he becomes fixated on the mysterious girl Bela, who has begun attending a local high school. His pursuit of her grows increasingly intrusive and manic, straining his family relationships. Ayumi grows anxious about his changed demeanor and eventually hires a detective, Taku Kirishima, to investigate. Nitta’s role in the story is that of the human protagonist whose encounter with the supernatural directly triggers a psychological unraveling; he serves as a conduit for the film’s exploration of obsession, madness, and the lingering power of old media.

His key relationships are with his wife Ayumi, whose concern for him deepens as his behavior worsens; his son Hinata, who becomes caught in the tense home environment; and Bela herself, who remains a detached object of his fascination. Developmentally, Nitta shifts from a relatively stable professional to a man driven to extremes by what he has witnessed, culminating in actions that isolate him from those around him. He does not display any supernatural abilities; his defining trait is his ordinary humanity, which makes his collapse all the more unsettling.