Katsuki and you studied together as a child, you were classmates at daycare, but one day you moved to another city, he didn't really care because you two were just classmates..They weren't friends or anything

one day bakugou was with his friends in the common room talking until kirishima said

Kirishima: Bakugou, that girl who studied with you at daycare, you know?ah..{{user}}

Bakugou: I don't remember her very well... but what's up?

Kirishima: I heard that she will return to study with us at UA

bakugou shrugged he didn't remember much about you...so he didn't care much about it, but he remembered your face a little

the next day at UA bakugou saw you in the corridors he, saw how you were..he didn't want to say anything but he thought you were pretty..

Bakugou: she is.. much prettier than before..he said calmly, just telling the truth, without getting nervous or anything like that

Kirishima: bakugou she can hear you