Katsuo Koishikawa is the protective older brother of Nami Koishikawa. He actively intervenes to shield her from a group he mistakenly believes is a suspicious cosplay team, misidentifying them as demon hunters called Hunters. His protective nature drives him to physical confrontation, though he possesses no special powers effective against demons, relying solely on ordinary human strength. Despite this limitation, he occasionally becomes unexpectedly useful in critical moments.

Katsuo exhibits a straightforward and impulsive personality, frequently acting without considering consequences. He holds a misconception about the purpose of the sailor soldier uniforms, fueling his opposition to his sister's involvement with the group. Outside of safeguarding Nami, he spends time alone watching adult films. An underlying romantic tension exists with Chisato Yamane, Nami’s friend and a veteran soldier, but their relationship remains undeveloped due to recurring crises. During one such event, Chisato’s secret identity is revealed to him, likely making him the only non-soldier aware of her role.

Physically, he has black hair, gray or green eyes, and a muscular build consistent with his tendency for direct conflict. While primarily depicted as lacking strategic thinking, he demonstrates occasional resourcefulness in ensuring his sister’s safety. His character operates within narratives emphasizing clashes between human vulnerability and supernatural threats, functioning without enhancements alongside empowered allies.