I think its better for u to find out about it yourself in this version because who knows they might change it But fine i can say the explanation in the novel. SPOILERS FOR THIS: YOU HAVE BEEN WARNED.
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He has a problem with his head fr fr. He is doing it with another girl for him to test his feelings. I know that doesn't make sense but it will be. Also, when he was doing it with that purple btc, yes he was feeling guilty but he needs to do it for the MC. If not MC might not love him anymore, that's what he thinks. In short, he is cheating on her because she loves her. Not only he did it once with that purple headed witch, he did 3 times in the novel. And when the MC starts to get fishy when that piece of sh*t keeps ignoring her, the ML KEEPS COMPARING HER to that purple headed btc. He was like "Why can't she be more like her." Blah blah blah. If u love someone, u need to have trust and communication. And that's what this guy's problem is. He is dumb, naive, and a piece of sh*t. Glad that he got what he deserves after the MC found out all about his cheating. And damnnn the MC WAS DEVOURING. I say, this is worth to read. Not because of the cheating part, BUT THE REVENGE IS SO SATISFYING. I really hope they wont ruin it on this version. The novel was really good to be just ruined.
OH THE ONE WITH GLASSES? His hair is blond Also, i got confused in a sec since there was no "Jae-Yun" in the novel. They probably changed the name, since the og name was supposed to be "Ji-Hyuk" And no. He is not going to be in the revenge part. Like i said, only the 2nd ML will help the MC. AND LIKE I ALSO SAID THAT THEY WERE A PERFECT DUO. I don't really ship them since they acted like siblings in the novel but it was mentioned that the 2nd ML is in love with our MC and its also stated that the ML and the 2nd ML are best friends back at elementary. But in the future they don't seem to know each other. But anyway that's all, i dont wanna spoil so much without a warning.
I have found the novel, it's so expensive. But it's worth reading, tho it's still ongoing. Anyway, yes the ML will cheat. Ik, it's heart breaking but i don't really want to spoil the rest so i just want to say that there will be an explanation. And if ur wondering what really happened in Chapter 1 why the MC is in jail, she GOT framed on something she didn't do. But she just accepted it, since it's her part of "revenge" against the ML. Even tho she knows that she got framed, she still uses this for her plan. And her plan was AMAZING, idk if they will change it in this version. But i gotta say, the MC was literally EATING SO MUCH in the novel. Btw, don't worry she will be free in jail. About the purple headed girl, be ready u might hate her a LOT lol. Also, yes. There will be a 2nd ML, him and the MC we're such a great duo for the revenge. And the ML, damn his chasing arc is so sad. I mean, it's his fault. That's all for this spoiler, but there will be more so hoping they will still add it in this version since the novel was so good even tho it's still ongoing. PLEASE DON'T DROP THIS IT WILL BE GOOD.