3 - Ups, he considered me as girlfriend? FUUUUUCK!
This is what I understood from her reaction: “So he considered me his girlfriend?”
I don't think she felt the same way about him, but now, seeing how well he's doing, seeing what she's lost, she'll start to wonder “what if?”, what if they were boyfriend and girlfriend, there's nothing wrong in that, they're great friends, the sex is great, he's a great guy... what if?
Seeing her arrive “late” in the game, I imagine that her role is to be a temptation to the MC, and a threat to the other girls, based on the prediction of the number of chapters the game should only be 60% complete I believe, so there is still a lot to happen.
She will try to get closer to him again, try to stick together like old times, comfort him when he has setbacks, she will be there for him if he encounters problems in the relationship (and MC is a crybaby, that will happen), and if they have sex again, she will show that she is also an option, she is interested, she will be a threat to the other girls.
Right now, that's the only role I see for her in the story.