He's a kid, let it slide.
Bruh I know a lot of kids but this isn't a normal behavior,,,, he's not even that young teenagers his age don't usually fucking force themselves on others who are definitely uninterested. This isn't him being a kid this is gross behavior.
He's 18 and he's not doing anything wrong to be honest. Asking somebody out on a date (Byul agreed!), trying to kiss them... It's not like he's harassing Byul or anything, so chill your bones. He's just a teen in love, that's it. He'll have to accept it that Byul isn't interested in him no matter what he does anyway, sooner or later.
He KNOWS he made Byul uncomfortable in the past, he comes back to his life playing as his little brother because he KNOWS Byul won't be okay with it if he tried to come back as a loved again. He KNOWS Byul is not interested and he still jumps in to kiss Byul without consent. It's not him innocently thinking his feelings are mutuals. They know they aren't, so he's trying to abuse out of Byul the most he can get. He already been rejected. This is manipulative af.
You sound like you've never been in love before.
You can't help whom you like, whom you're attracted to. The little brother sure has been rejected, but he's not past that stage yet where his mind caught up with this fact. People need time, you know. If you give up immediately on your feelings and move on like nothing ever happened as soon as someone tells you they're not interested, then I don't really know how to explain it to you.
He's not being manipulitive, he's just being helpless. This drama he is stiring now will fade away in the end anyway.
Sorry, I meant
*he's not in that stage yet where his mind caught up with this fact
Oh honey you do not know a thing about me. I have been inlove, I have been in situations where your feelings take over you. But you know what? You might not be able to control your feelings, but you have full control over your actions. Your feelings might effect your judgment, but you control your actions. Forcing yourself on someone is NEVER okay, I don't care how inlove you are. He IS being manipulative. I can understand and excuse him trying the most to stay close to Byul because he can't get over him. What I can't excuse, and could never get behind, is him actively doing stuff to hurt Byul feelings, and using his brother status to abuse stuff out of Byul. He is HUTING Byul. That's him being a terrible selfish brother, not him being inlove. Usually when you're inlove, you respect the person you love's feelings, and do the most you can to not hurt them. If this is your definition of being in love, then I'm sorry, but YOU have never been inlove.
How can his brother be so terrible if I was rejected like that i'd be so fucking ashamed he forced himself on his goddamn brother knowing he's not interested in him that's so gross how can he even live with himself