Sanae Mizuno is a nurse at Yomiyama Hospital who appears early in the story when she is assigned to care for the protagonist Kouichi Sakakibara during his hospital stay. She is an adult woman with short brown hair and brown eyes, and she wears a nurse’s uniform. Her younger brother, Takeru Mizuno, is a ninth-grade student in Class 3‑3 at Yomiyama North Middle School, the same class Kouichi later joins. Sanae is aware of the curse that haunts Class 3‑3 and shows concern for her brother’s safety. She is kind and professional toward Kouichi, and she tries to assist him in understanding the strange events surrounding the class. She shares information about the hospital and the town, and she expresses worry about Takeru’s involvement. Despite her helpful nature, Sanae becomes a victim of the calamity herself: while on a phone call with Kouichi in an elevator, the cable snaps, causing the elevator to plummet to the basement. She dies upon impact and is buried under debris. Her death underscores the reach of the curse beyond the classroom, affecting even relatives of Class 3‑3 students. Sanae does not possess any supernatural abilities; her role is that of a supportive secondary character whose fate illustrates the inescapable nature of the tragedy. Her relationship with her brother is strained, with Sanae describing Takeru as not particularly intellectual and showing frustration over his lack of interest in serious matters. Nevertheless, her motivation is rooted in protecting him and helping Kouichi, which ultimately leads to her untimely end.