The drama between her, is she the one that he will end up with or she will be the ultimate...

I think she was supposed to be the fl, but because of this conflict they had a falling out and the mc will end up with the boss in the main story.
It has a "what if stories" and in each of them the mc will end up with other girls.
Actually, from the beginning it was obvious that she will be the fl but the author made a reason for it not to happen. They have the perfect cliche story, from enemies to friends to lover and there is angst in it as well, but sadly they couldn't overcome it in the main story.