Kōichi Nimura is a literature freshman at Tama University and a member of the Street Observation Research Society alongside Yuuta Segawa, under Shuntarō Sako's leadership. His friendship with Yuuta blossoms through club activities. Preferring not to stay at his own home, which he describes as exclusively for taking girls, Nimura becomes a frequent presence at Yuuta's residence. This habit makes him a fixture in Yuuta's life, especially after Yuuta takes custody of his three nieces.

Nimura consistently supports Yuuta, voluntarily tackling household chores and babysitting responsibilities. He helps Yuuta manage the challenges of guardianship, even intervening to prevent Shuntarō Sako's inappropriate advances toward the nieces. His reliability shows in prioritizing friends over romance; he readily abandons dates if a friend needs help. Though characterized as a womanizer often seen with new partners, he remains uninterested in fellow club member Raika Oda, influenced by Yuuta's feelings for her and Raika's unconventional personality.

He shares a particular rapport and strong mutual understanding with ten-year-old niece Miu Takanashi. Nimura helps Miu navigate emotional challenges, like overcoming depression stemming from classmates pitying her family situation. His actions across the series and its manga spinoffs, including *Rojou Kansatsu Kenkyuu Nisshi* and *Miu-sama no Iu Toori*, consistently reinforce his role as a dependable friend and supportive figure within Yuuta's extended circle.