Childhood friend of Kogami Kii.
10 years ago arc
When Kogami enters her room, he finds disgusting looking drawings of these mahou shoujos in this box she kept, which therefore proves that she has something to do with this whole thing. After she awakes in her fathers hospital Tsukune's true personality shows as being sadistic, vulgar, and unstable. It's also hinted that she personally killed Sayano's parents and was threatening to do the same to Sayano all throughout high school.