You're telling the dev who it is?
I'll bet you are wrong though.
Bella and Monica are far too obvious.
Just because something is obvious doesn't mean it's bad storytelling. People need to stop associating being unpredictable with good storytelling. Anyone can be unpredictable in their storytelling, that doesn't mean it's good. Especially this far into the story you want your threads to connect and make sense. Also people make fun of tropes, but the reason why things are tropes is because tropes make sense. And making sense imo is good storytelling.
And imo the most unpredictable thing would be if it was Monica. Everyone can have their personal favorite girl, but Monica is clearly the Main Girl, she's the first one you see, and you still haven't banged her. So if it is her? That would be very surprising, but also be like, what was the point of the story?
It would be like in story like Legend Of Zelda, and Link tries to help Zelda, but then we find out in the end Zelda is the final boss.
So for that reason I don't think it's Monica.