Mizuki Amatsuka, the fifteenth daughter of the Amatsuka family, is four years old and attends middle class at kindergarten. She dresses in the traditional attire of a Shinto shrine maiden, a miko, and possesses a nine-tailed fox spirit as her pet. Mizuki speaks using the distinctive Kansai dialect of Japanese. She holds deep admiration for her adoptive brother Yōtarō, sensing a kindred aura from him that forms the foundation of her respect and affection. Due to her young age, she required assistance during a family river outing, needing to be carried to the bathroom. Within the family hierarchy of nineteen sisters, her position falls between the fourteenth daughter, five-year-old Mari, and the sixteenth daughter, Sakura.