SPOILERS ON WHO IS THE MALE LEAD

Spoilers below. This is only based on the korean raws. So vague impressions and understanding.
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In the main story the ML is the ex boyfriend. It's pretty straightforward that the ML and FL is the endgame coz they both still have feelings for each other. And ML is possesive over FL and has not been with/dated any other girl/ since he met her. It just took a while to fix their misunderstandings and get back together. But that did not stop them from being intimate. So many many sexy times.

As for second ML the sexy scenes with him were ALL only in his imagination and one time the FL's imagination. And a kiss when the second ML finally confessed to her but FL stopped it and did not go any further. He does seem to end up with her in the side stories. It seemed like an AU if FL and ML did not end up together. Coz ML is still there and likes FL but FL seems to reciprocate second ML more in that AU.

As for the short hair girl, she is not the ML's girlfriend. Just a friend. And is the lesbian love interest of the FL's sister (one of the twin siblings of the FL). She did bring about the misunderstanding between the ML and FL coz she was mad. Did not understand if she was mad at FL's sister or at FL for breaking up with the ML.