I think it's funny how everyone instantly assume Kei is a boy crossdressing as a girl just because of the tag, but there is a much simpler alternative: Kei is a girl crossdressing as a boy when she was younger. Also, we didn't even know if Kei know that MC liked Mitani, though I assume Kei lifted Mitani's skirt to give him "material" since Kei was teaching him.
Sure you can prove me right/wrong since this has a lot of chapters already, but this is my first time reading this.