Yumiko Ouse is Kubo Masaru's classmate and Mizushima Akira's childhood friend. She possesses long, straight brown hair and is typically seen in her school uniform, complete with a hair ribbon, shirt, and skirt. Her personality is friendly, kind, refined, relaxed, and reserved.

During childhood, a traffic accident claimed both her parents. Following this loss, she moved in with her kind and gentle uncle, a teacher at Kashima School, who consistently provides support. This situation defines her as an orphan under her uncle's care.

Yumiko suffers from chronic asthma, leading to frequent sudden attacks. This condition regularly forces her absence from school and impacts her daily life and education.

Within the narrative, she interacts with Masaru during his rehabilitation after a career-ending boxing injury, meeting him initially at the sanatorium he visits for recovery. Later, she is present at the summerhouse during the incident involving Akira's kidnapping.

Yumiko participates in cooking activities. Her key relationships center on her connections to Masaru and Akira within their school environment.