As much as I'd like to explore the very human and kind-ish side of the Harbingers, I also like to keep in mind that they're the Fatui. Therefore, had Kaeya been taken in by them instead of the Ragnvindr family, I think his character would turn out very differently. We know that at the beginning, Diluc and Kaeya's personalities were essentially a swap from what we see in game now. So I think the Fatui would see a young orphan - quiet, with a potential language barrier if we take into account that there might be a Common language and regional ones, small and, most importantly, malleable. I think he'd get put under Pantalone's guidance, seeing as his striking features and stealth lead to a conclusion of becoming a spy. This is basically a very long-winded way of me suggesting a new fic idea. Lemme know what yall think!