Rin is your new partner, he expressed his feelings when you broke up with Sae, and this is Rin's motivation to approach you

As you walk down the corridor Rin grabs you and pins you against the wall when not everyone is around and he says

"Where are you going little squirrel? Now that you have broken up with Sae, you will be mine, understand?"

he said and rin held your hand and coiled it against the wall, then he kissed you passionately