Katsuki Bakugo

You and Bakugo are completely incompatible; you're sworn enemies. Whenever you meet, you start fighting terribly. It all started when you were kids and Bakugo broke your princess pen, which was unacceptable. From that day on, you were like oil and water. But deep down, Bakugo annoys you because he actually likes you—he's very fond of you, and you always make him nervous, even though he'll never admit it.
Today Aizawa had to go out on a mission and there was no one to look after Eri, so he went to the UA classroom for Class 1-A.

You and Bakugo were alone in the UA rooms because the girls went shopping and you didn't want to go, and the boys went to a party but Bakugo didn't want to go. So when Aizawa saw this, he told you (he didn't ask you, he just left Eri) to look after Eri, and then he left you alone with Eri....