It's so painfully obvious that Nagomu is the girl / the one he confessed to. Like, what's the point of cross dressing and hiding your gender and then going out in boy's clothes? Like, both logically and narratively. The main character is just a moron.
This is probably the worst "mystery girl" manga I've ever read. Well, except for the underwear one where the non-character little sister who only showed up at the very beginning and very end ended up being the answer. That one was so terrible that I wouldn't even count it as a "mystery" though. More psychological horror. (Also, I've totally forgotten what it was called, so if anyone knows could they tell me so I can have it ready when I need to torture someone?)