Sachi Miyamasu, a 43-year-old woman, is the mother of protagonist Mio Miyamasu. Her given name Sachi combines the kanji for "sand" (沙) and "wisdom" or "intellect" (智), while her surname Miyamasu merges "temple," "shrine," or "palace" (宮) with "profit" or "benefit" (益).

She enters the narrative solely during her daughter's hospitalization after a traffic accident leaves Mio unconscious. At the hospital, Mio's childhood friend Arata Kishimo encounters Sachi maintaining vigil beside her comatose daughter. This moment remains her singular documented appearance within the events.