mist!!

My first thought when I saw this scene was, "Wait, isn't that Kyo's hair?" (source for Kyo's name). And yeah, when compared side-by-side, it certainly does look like the same hair! So, new theory: Kyo was adopted by Makoto or Komaru (or perhaps a child or grandchild of one of them, considering the span of time between canon DR and DRDT), and is also a trans girl, but didn't figure that out until after getting adopted, which is why we/Teruko know Kyo as a boy.

This also would give some context to the caption for the au drawing, which is, "In the scenario where Teruko and Kyo stayed together. There are also changes to the lives of other cast members, but, I’m too lazy to draw…" Perhaps this means that Kyo is scouting Ultimates in the US to try and look for/reconnect with Teruko, but if they had stayed together as children, Kyo wouldn't have been doing that, so Ace and some of the other students may not have been scouted.