Kōtarō Dōchin is a 13-year-old human middle school student with no living relatives after his grandfather's death. He is taken in by the demon detective Akutabe, an old friend of his grandfather, as explicitly stated in the manga. The anime presents an alternate background where Akutabe claims a debt owed by someone else motivated taking him in, attributed to production constraints. Dōchin displays mischievous and mean-spirited tendencies, frequently plotting against Akutabe, though his plans typically fail due to his own lack of foresight and intelligence. He adopts a facade of innocence to manipulate others, easily seen through. He possesses a strong fascination with voyeurism, particularly involving naked women, which often distracts him and serves as a recurring comedic element. He tends to complain about the lack of adult support for his problems, despite being their primary cause.

Physically, he appears as a young boy with black, spiky hair, commonly wearing his middle school uniform. His attempts to project an innocent look often fail to conceal his true nature. As a demon contractor, he can see, interact with, and summon demons. He formed a contract with the demon Gusion by sacrificing his grandfather's memories using a high-ranked magical grimoire inherited from him. Gusion, one of Solomon's 72 demons resembling a winged monkey, possesses the "Forgetfulness" ability to consume others' memories, particularly fond ones, and represents the sin of Sloth.

His relationships involve conflict and manipulation. He sexually harassed Rinko Sakuma upon their first meeting, leading to her initial dislike. Despite this, they develop a coexistence where she is strict with him regarding his voyeurism, occasionally relaxing together at the agency. His relationship with Azazel is antagonistic; he picks fights with the demon and exhibits a parallel personality in anger and verbal abuse, though lacking Azazel's perverse humor. He resides with Akutabe at the detective agency in the manga continuity. Dōchin first appeared in Volume 2, Chapter 19 of the manga and Season 1, Episode 7 of the anime.