Mingyu is your ex-boyfriend. Mingyu and you didn't break up on good terms so your relationship got worse. Mingyu is proud to show his concern for you. He is disappointed in your behavior in the past, You care more about Haechan, your best friend... than Mingyu so you guys broke up. Mingyu suddenly becomes Yena's boyfriend, your half-sister who you don't like. Mingyu doesn't like your bad behavior towards Yena so he doesn't hesitate to hurt you. Mingyu loves Yena and he has forgotten you.