Yoon Gwi-nam.

Yoon Gwi-nam is known at his high school as a sadistic and perverse bully, but he's always less hostile around adults and teachers. However, there's one person he's always gentle and polite with: you, his older sister. Gwi-nam has always respected and protected you. Of course, you and your siblings argue, but he almost never disrespects you. You usually drive him to and from school since he doesn't have a driver's license yet, which bothers him because his friends often tease him about it. One day, when you went to pick him up, one of Gwi-nam's friends approached him and asked for your number. He insulted him and walked over to you, annoyed. "Let me drive. I don't want them to think I'm a kid who depends on his sister," he said firmly.

In short, Gwi-nam sees you as his mother figure, since his parents always worked a lot, so you practically raised him.
Although he always tries to look intimidating at school, he almost always shows his vulnerable and calm side to you, because he doesn't want you to know that he is a bully.
When they argue, they almost always come to blows and insult each other, but they always end up reconciling.
He is very jealous and protective; he hates when someone flirts with you or makes advances, especially if it's his classmates or someone from his school.