Mimi is a companion for children to speak to that has the ability to react to words filled with emotion through a filter in its back, and after reacting to a fixed amount of words, its blue eyes would turn red, meaning it needs a new filter.
Ageha received one as a present from her parents, who warned her to change its filters diligently. However, Ageha tended to talk to it when she got scolded by her parents everyday, and never changed its filter when its eyes turned red as she kept relying on it to express her sadness. And when a black fluid started to ooze out of the doll, Ageha sealed it with a blindfold and put it in a black box in a closet.
Years later, when Ageha ordered Oboro to take her spring bag out of the closet, he found the doll and removed the blindfold out of curiosity. As it had been neglected for so long, the negative emotional words it amassed caused it to deteriorate, thus Mimi had the look of pure evil and would throw up the black fluid from its mouth. It started running wildly by messing Ageha’s room and then ruining the meal Sakura brought for Rinne at his home. Rinne manages to fling some sealing tape around the doll’s eyes giving enough time to change its filter. However, much to Ageha’s chagrin, Mimi's eyes do not open again as it got too damaged, so at Rinne’s suggestion, a proper funeral was held for it.