Grill will leave to train to be equal to MC (cuz from her and everyone's POV in this world MC is fucking OP already). You know, because for some heroines, being only a sex slave of MC is not enough. Just the opposite of what
@kronix said.
Fun fact, when someone will try saying something to Eris along the line of "you became this strong by sharing a bed with your sword teacher", he will get his ass handed to him pretty roughly. I mean he will die.