I commented this in an earlier chapter, but I'll say it again.
I'm pretty sure Mitani is a typical girl that's all "If I can't have him, no one can, and he can't have anyone, and I'll make sure that's how it stays by any means necessary. Even if it means tricking someone he's interested in to fall for me, just so I can play with their feelings while keeping them away from him."
TL;DR: "If I can't have Boy Kei, no one can have Kei, and Kei can't have anyone. I'll manipulate and play with the feelings of anyone who shows interest in Kei."