Ewww yea i really hope not. Even if it's fantasy and they're mythical creatures, that age gap when they first met is just ick. If Uijin was an adult like Hajin at that time, it would at least be a bit considerable.
But the overall backstory could never justify the romance behind it if they're really a couple. For now I see their dynamic as a betrayed child who's looking for explanation from the adult he once trusted. But I'd brand him a traitor if he really falls for someone who found it amusing to induce a massacre in their village & never took accountability for it

From a few chapters before there were comments saying that the author confirmed that Lee Saha and Uijin aren’t a couple, but now I can’t find those comments and the closest comment I got that mentioned that they aren’t gonna be a couple is this:
https://www.mangago.me/home/mangatopic/22943032/
I’m just uncomfortable with the fact that Uijin, as a kid, was really close to Lee Saha, and if they become the second couple and when it becomes their time to “shine”, I’m not gonna read their part lol It just doesn’t sit right with me, but I’m not gonna judge y’all that like that trope tho, I’ve got my fair share of liking questionable tropes.