he has trauma, i think the story was pretty clear about how that's his coping mechanism ????? he literally self-sabotages himself with that personality, because he doesn't want anyone to get close enough to hurt him. his parents were shit to him, and when he thought his best friend had his back, he found put that they didn't actually care about him.
ofc you're allowed to not like a character because of that behavior, but you're completely ignoring the nuance ? or at the very least based off of how you wrote your comment. Heejae isn't completely faultless either, he constantly pushes Seyeong's boundaries despite numerous verbal and physical rejections. at the core of their characters, Seyeong is an extremely vulnerable and emotional person who is selfless but acts selfishly. Heejae on the other hand is also a vulnerable and emotional person, who also has emotional scars just like Seyeong, but he is a selfish person who acts selflessly. neither of which are a bad thing.
i love both characters a lot, so it hurts to see people dismiss one completely, especially one i empathize with and see my friends in. hopefully you don't dismiss this as another "can't stand a different opinion" thing, because i can; as long as it is made with the proper knowledge and nuance and not just off of vibes.

I’ll never understand the appeal of characters like MC. He’s got a vile nasty mouth, is always violent, and acts before he thinks. These are all super unattractive attributes, his good qualities are out weighted by the bad ones, making him an unbalanced romance protagonist.
He’s just purely annoying, it’s fucking tiring
It would’ve been easier to ignore if it was done in moderation, but it’s his entire personality, massively overdone. The way he acts isn’t cute one bit.
The rest of the plot is fine, definitely very overhyped