Mio Imai is Satsuki's friend and classmate that appeared in The Corpse that Roams in the Night – Shirotabi. She is a shy girl that was friend of her bunny, Shirotabi. Shirotabi died, and Mio became really sad. So, she made a spell in order to Shirotabi come back to life, however, Shirotabi came in the form of a monster. In the end, Satsuki and her friends managed to exorcizate Shirotabi, and, despite Mio being sad, she was kinda happy as now her friend could rest in peace. She also likes to make animal dolls, and even gave one to Satsuki.

(Source: Gakkou No Kaidan Wiki)