You wanna waste a card on Faceless McSideCharacter, the background, stupid, terrified schoolgirl? Really? Why?
[X] You keep her heart until you're clear. You don't really trust her not to do something idiotic just at the moment. On the other hand, you don't really give an honest shit about this girl, and anything you can force out of her, you can get almost as easily by handing it back with a smile, later. All else considered, you're already coming out way ahead.
Let's see, let's see.......
[X] Is Mami any good at marbles?
-[X] If not, attempt to make a trade. Be ballsy. Offer to trade a unique thing that can only be given once, your own virginity, to be given to her directly. There's no way that can go wrong!
--[X] When it inevitably goes wrong, play a few games of marbles, only wagering things you truly don't care about losing. Schoolgirl isn't on the list. You plan on giving her friend the hugest shit eating grin when she hears that you gallantly saved her, where she broke and fled.