It can be any sort of drama, but the the romance aspects deal with the fact that the FL is not over another guy. She wants to be with the ML, but lingering feelings keep occurring. That causes many problems for her relationship with the ML. She continuously hurts him and feels guilty about it. Maybe she dated the other guy, but they broke up with her. Maybe it was just an unrequited love. Maybe, the other guy starts to pursue her since getting with the ML. The ML & FL reconcile in the end and stay together.
Like the second half of Love Buffet.