Like others have said, I hope this new girl gives Megumi a kick in the butt. She's not here to steal Manabu, but she is here to probably make the FMC step up her game. Then again, our MMC is kinda dumb (when it comes to relationships) but who wasn't at his age. One of the two between our MCs needs to push forward and Manabu has never really given it too much thought. I doubt that Manabu has ever really seriously thought about Megumi (romantically not sexually, because he does that ALL THE TIME) like he does the Tokyo-U girl. He's so focused on the girl he doesn't have (but wants) that he can't see the girl he DOES have (but doesn't acknowledge that he wants).
Between the extended stay-over by Megumi, this new girl, and hopefully some kinda realization from Manabu something should give. But I'm NOT getting my hopes up, LOL.