Mamiko Ōmae is the elder sister of Kumiko Ōmae and daughter of Kentarou and Akiko Ōmae, residing with her family near Byōdō-in Temple in Uji, Kyoto. Her middle school euphonium playing initially inspired Kumiko's musical pursuits, but Mamiko abandoned band in high school to prioritize college entrance exams—a choice she later regretted due to unresolved parental expectations.

During Kumiko's first year at Kitauji High School, Mamiko attended college. Tensions erupted when she expressed a desire to drop out and train as a beautician, overheard by Kumiko during a family dispute. This revelation sparked friction, culminating in an emotional confrontation when Mamiko interrupted Kumiko and Reina Kousaka listening to a euphonium CD, where Kumiko lashed out. The conflict stemmed from Mamiko's lingering regrets over quitting music and perceived parental pressure shaping her choices. After Shūichi Tsukamoto revealed Kumiko's longstanding wish for her to hear a performance, Mamiko began reflecting.

Mamiko reconciled with Kumiko during a typhoon. While preparing dinner together, they confessed mutual jealousy—Kumiko envied Mamiko's academic freedom, while Mamiko envied Kumiko's continued musical dedication. This exchange prompted Mamiko to confront her own unresolved aspirations. Her development culminated in attending the national band competition to support Kumiko, leading to an emotional reunion after Kitauji secured bronze. This act symbolized her acceptance of Kumiko's path and closure regarding her abandoned musical past.

Later, during Kumiko's third year of high school, Mamiko provided guidance as Kumiko faced career uncertainty. Their father advised Kumiko to consult Mamiko, reflecting her trusted role in post-secondary decisions.