Kōichi Nimura, a first-year literature student at Tama University, joins the Street Observation Research Society and quickly forges a close friendship with fellow member Yuuta Segawa. Openly identifying as a womanizer, Nimura claims his own apartment exists solely for entertaining female companions, leading him to spend minimal time there; he is frequently found at Yuuta's place or with romantic partners. Despite this lifestyle, he demonstrates authentic care as a friend, often voluntarily assisting Yuuta with household chores without prompting. His loyalty extends to actively helping Yuuta protect his young nieces, notably aiding in keeping Shuntarou Sako's inappropriate advances toward them in check.

Nimura develops a notable bond with Yuuta's ten-year-old niece, Miu Takanashi, taking her on a date and sharing mutual affection, with Miu appreciating his presence. While his interactions reveal a protective side toward the family, he exhibits no romantic interest in another club member, Raika Oda, potentially out of respect for Yuuta's feelings for her. Nimura consistently prioritizes aiding Yuuta and the girls over his social pursuits when necessary. His character remains stable across official media, sustaining his role as a dependable comrade who balances his womanizing tendencies with steadfast support for the family unit.