It was hinted at the start, so it isn't really that random. Remember when Yahwi already knew where Jooin lives when he was a kid and then the picture of them together after that.
ohh i think i know what you’re talking about. i dunno.. i’m still sort of off put by it since it feels like it was suddenly introduced Σ( ￣□￣||)
Ooh for me, I really wanted to know how Yahwi knows Jooin and what Cain's past is; how does he know Jooin and why does Yahwi dislike him to the point he calls him such words, so this is the half of the part I've been waiting for. ╮(￣▽￣)╭
the childhood friends trope felt kinda random to me...