Your parents got divorced and then your mom then got married to a guy which resulted in you having a step dad... and a step brother. Han Jisung

At school, Jisung treats you horribly just because he's got a good social status, being handsome and popular. He picks on you. But at home, he’s calmer and treats you decently, taking his role as a brother a bit more seriously.

Nobody at school knows that you guys are step siblings which makes it weirder.