Spoiler
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I didn't like him in the beginning too, but turns out he wasn't cheating at all. And the one who was putting money secretly in her pocket during her high-school days to help her was him not his brother. And he's basically been yearning for her for his whole life. He was happy he could be the one who gets to marry her, but he thought she was in love with his older brother. He was also doing the cheating scandals on purpose to throw off his family who was secretly behind FL's downfall. He was plotting revenge on his family for her. And also kind of for himself.
FL is also pretty cool, like yeah at the beginning she seems like she kept letting all the supposed "cheating" happen, but she never let him into her heart for real because her father cheated on her mom, and she was blamed for being born, so she would never let it happen again. She only ever really loved him until after he cleared up all his fake cheating. So, she's not a pushover.
Welp... there's a couple reasons for that...
1, he really believed she was in love with his brother and was forced to marry him and didn't want her to feel pressured by the marriage... that's why they kept it a secret too...
2, to stave off his family's attention... it's not very clear at first but they basically kept them under surveillance and guarded a lot against ml... so acting frivolous and playboy like was to lower their guards until he was ready to confront them.
3, the one with the most scandals, the red head was a willing cover that got greedy and the reason he atured her antics was because he felt indebted to her mother... she helped in in hard times when he was younger.
So yeah... same with her acting like she was in love with his brother... she was thankful cuz she believed he was the one that gave her money but she didn't really love him... she acted such so ml's stepmother would believed she was easy to deal with and not suspect and guard too much against her...

i mean wtf with the early chapter