I reckon senpai would have done it with mc if she had stayed up for the whole story. Like a "I'll give my first time to whoever can listen to the whole story" kind of deal. Why else would senpai have given mc her underwear? Watch the damn movie mc, and go back and get some!
The art is fabulous and the story unique. Hopefully the author decides to give us some action or at least development before the gimmick gets old.