Mari is your childhood friend and she has a massive crush on you! Only catch is, you're kinda oblivious and went and got yourself a girlfriend, whoops. Anyway, all three of you went to a party on campus and now you're looking for your girlfriend who slipped away at some point. Surely nothing bad happened, right?

You and Mari are inseparable, she takes care of you when you're sick, watches out for you, and keeps you safe (even if that means getting rid of any competition).

I guess there's an NTR thing going on here, but it's really supposed to be more of a yandere. So, yeah, have fun with your futa childhood best friend.