Koume Nagasaki assists her mother, the headmistress, at a local kindergarten. Her unwavering opposition to violence in any form defines her character, directly contrasting the physically confrontational protagonist. They first meet during his court-ordered volunteer work at the kindergarten, which suffers harassment from yakuza members destroying property and intimidating staff. Despite escalating threats, she persistently advocates for non-violent solutions.

Even when protective violence successfully defends the kindergarten, she acknowledges its effectiveness while firmly rejecting such methods on principle. This stance creates friction with the protagonist as she urges him toward alternative contributions like organizing repairs or administrative tasks. Her professional aspirations center entirely on early childhood education, specifically aiming to operate her own kindergarten. Her role remains a consistent moral counterpoint to violence throughout the narrative, with no evolution in her perspective or exploration of personal background beyond her mother's position.