Kōichi Nimura is a university freshman at Tama University's School of Literature, where he studies Japanese literature. He is a handsome young man with a slender build, silky, messy brown hair, and a sweet, approachable face. His personality is light-hearted, easy-going, and openly womanizing; he cheerfully describes himself as someone who only brings girls to his own home, a habit that leads him to spend almost no time there. Instead, he frequently crashes at his close friend Yuuta Segawa's apartment, often helping himself to the space and food.

Beneath his carefree, flirtatious exterior, Kōichi is a genuinely caring and dependable friend. He willingly takes on household chores without being asked, and he and Yuuta have a mutually supportive relationship built on casual banter and practical aid. They call each other Segawa-chan and Nimura, and they cover for one another with class attendance, part-time job scheduling, and other everyday problems. Kōichi is also a member of the Street Observation Research Society, a university club that provides a social setting for many of the story's characters.

Within the narrative, Kōichi serves primarily as a supporting character and a source of comic relief. He provides a contrast to Yuuta's sudden, serious responsibility as a guardian, and he often offers lighthearted advice or teasing remarks. Despite his womanizing reputation, he shows genuine concern for Yuuta's well-being and the situation of the three sisters. His character does not undergo major development, but his loyalty and willingness to lend a hand are consistent traits that reinforce the theme of friendship and found family in the series.