This article contains descriptions and images relating to plot points from the anime.
Sempai's older Sister (お姉ちゃん Onee-chan) is Assistant's homeroom teacher. She adores Sempai and is very affectionate towards her. Onee-chan often teases and embarrasses Sempai, and tends to call her "precious" and "Little Sis", which Sempai finds annoying. She also likes to dress her sister in costumes. Onee-chan is married to a man named Mimura.