Miki Saashita (坂下ミキ Sakashita Miki) is the 14-year-old cousin of Yuzuyu Sakashita on her father's side. Volume 4 of the manga is mostly Miki's story. In the manga, she carries a knife, which was changed to a bike chain in the anime. Miki attended a prestigious school where she witnessed a teacher beating a fellow classmate. When she confronted her teacher, everything backfired. She was harassed by her peers and teacher, but refused to tell her parents. Kippei Katakura notices the burns and scars on her wrists, indicating that Miki engaged in self-harm. She tries to take Yuzuyu away from the Katakura residence so that her parents will have a substitute for her when she attempts suicide, which Kippei prevents. (In the anime she intends to jump from a building, in the manga she uses a knife.) In the end, the issues between her and her parents are resolved. She appears again in Volume 6.
- Miki's surname Sakashita means "slope, hill" (坂) (saka) and "under, below" (下) (shita).