She doesn't like him, she likes Kei, but she wants the boy Kei, and if she can't have the boy Kei, then no one can have Kei, and if she can't have boy Kei ever, then Kei can't have anyone ever, so she plays with Yo's mind to make him think she likes her, all so she can keep Kei from getting Yo.
TL;DR: Mitani wants boy Kei, but if she can't have him, then no one can have Kei, and Kei can't have anyone else, so she keeps people away from getting close to Kei.