It's very clear from the beginning that it was the ML the FL had a one night stand with years before. The ML tried to snarkily jog her memory, but since she was drunk and the look and attitude of the ML now have become so different - she really doesn't remember that it was him. The ML is pissed off and in the beginning it's obvious to us as the reader that he's trying to get back at her. They get into a casual relationship but then ML brings her on dates, makes her feel special, as if the relationship is more than just physical. FL starts falling for him even after finding out the truth but ML is still very clearly annoyed at her and says some heartless things to demean her feelings, hence the need for him to win her back once she's finally had enough.

I will say though, the way the story played out, it does feel as if they failed to address a few of the points of their past which were very frustrating.

They never had a real conversation to clear up some of the misunderstandings.

When the FL found out about his identity, she felt betrayed. Then the ML said they can just pretend to forget about it and continue what they're doing. He was constantly like: "what's the big deal?" Maybe things would have been different if he had confronted her and the FL could explain her side of the story. Because honestly, besides leaving money behind, I don't see what the FL did as something that he needed to get revenge on. Of course she wouldn't remember! She was drunk - she didn't even want to remember!

ML was obviously affected by her not remembering their true first time meeting but they were constantly interrupted and the conversation never ended up happening. We were just fed a flashback and that was it.

One of the mean things he had told her was "if you want to date then whatever, but marriage would be difficult" and I honestly wish the author had explored that before suddenly letting us know that two years after she forgave him, they got married. Also, why couldn't we have a gotten a proposal scene?