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Izuku and Monoma have been secretly together since before either of them even got into UA. During the 2nd-to-last year of middle school, Bakugo did the same things as he did in their (canon) last year of middle school. But unlike Canon, Izuku told his mother, Inko, about it. Now, during all this time, his father, Hisashi Midoriya, had been abroad on business in America, but over the last year, he had moved closer, now being stationed in South Korea. So Izuku got transferred to a school in South Korea for his last year of middle school, but still wanted to go to UA. As luck would have it, he was a late bloomer and unlocked his quirk in the years he wasn't in Japan.
But where does Monoma come in? Right.
Neito's father is friends with Izuku's father, business associates, if you will. Izuku and Neito were friends even before he left for Korea, but Izuku didn't think he'd see him so soon. Shortly after Middle school, they both were basically inseparable.
(UA is a college/University in this AU)
Somewhere during high school, they both dated, long distance for the most part, since Izuku was still in Korea, but sometimes Monoma would come over with his dad when there was some kind of business meeting, which was usually once or twice a month for a few days.