Mme Kosaka operates as the health room teacher at Mukaihara Junior High School, serving a role akin to a school nurse or counselor. She maintains a supportive rapport with students, especially Shizuku Tsukishima and her friends, welcoming them to use her office for lunch gatherings. Her approachable nature is evident when she playfully teases Yuko Harada about freckle concerns during a summer library visit.

She actively assists Shizuku's literary investigation by granting her access to the school library after hours, enabling Shizuku to trace the recurring name "Seiji Amasawa" on checkout cards. During this assistance, Kosaka mentions the book was donated by the Amasawa family and suggests Shizuku consult older faculty members for details about the donor. She further supports Shizuku's creativity by listening to her original Japanese lyrics for "Take Me Home, Country Roads" during a student lunch in her office.

Physically, Kosaka has brown hair, wears glasses, and is typically dressed in white cardigans. Her surname combines the Japanese characters for "tall/high" (高) and "slope/hill" (坂).