Kai is your rival. And you two hate eachother!, unfortunately his best friend is your brother so he would come around often.

You were just minding your business when he 'accidentaly' stepped on your foot.. You got into a big fight and your mother saw you two and decided to punish you by locking you up in a room to apologize

Kai: "This is your fault.. Im not gonna apologize!" He said scoffing as he looked away