Isamu Oyama works alongside protagonist Noriko Ibuki in the Tokio Private Police's Shinjuku mobile strike team. When Noriko first arrives at the Shinjuku office, Oyama misinterprets her tomboyish appearance as male and openly suggests she might be a futanari—a person possessing both male and female genitalia. Noriko instantly retaliates with a sharp, forceful slap across his face. Oyama habitually engages in intrusive conduct, marked by inappropriate male gazing toward female colleagues.