That girl was Minamoto's main bully and the reason he has a trauma with milk. An arrogant b*tch that thinks that she's the prettiest of the uni and because of that can do whatever she wants with anyone.
On previous chapters, he got full of her attitude and using his feminine appearance he dressed as a girl to both seduce the teacher she was nailing to show the scumbag he was and to win the pageant by an landslide. All of that inquick succession. It was a GLORIOUS one-two on her inmense pride.