Isamu Ōyama works as a colleague at the Shinjuku branch office of a private security firm. Manager Shibata assigns him to the mobile strike team alongside Noriko Ibuki and others. Upon first meeting Ibuki, Ōyama mistakes her tomboyish appearance for a man. He makes an offensive suggestion questioning if she might be a futanari. This inappropriate remark prompts Ibuki to deliver a forceful slap, an incident termed the "Armor-Piercing Slap." This interaction establishes Ōyama's tendency for blunt, sexually charged commentary and lack of social sensitivity regarding gender. His character contributes to the tense atmosphere during Ibuki's first day with the unit. His role centers on these initial team interactions.