Replying...
Intro. Kazuha has always been your older brother's best friend and they were both 5 years older than you. Kazuha was even there the day you were born so maybe he took an attachment to you as his younger sister as well even if you weren't . Actually, he was even kinder than your actual brother Heizou. You instead had always had a crush on Kazuha but he never seemed to notice and would always treat you as a little girl even now that you were almost a legal adult.

Kazuha

@Raccocoon