My idea:

My theory is that Aaron and yujin met when they were kids. There's a flashback yujin has where he sees a boy with blonde hair and thinks something like "rich kids" and there's a scene where Aaron has a memory of a boy with black hair. I think it's safe to say, and as a pretty typical trope, they knew eachother when they were young. Also I'm the first or second chapter Aaron tells Yujin that he knows he used to want to be a singer.

I also think that yujin saved Aaron from yujins dad when he was younger. In the first few chapters when they are in that gun room Aaron says it's because he was shot at when he was younger and he has them to protect himself. He says that yujin looks like the guy who shot at him. In a chapter shortly after there's a scene of a man with black hair watching Aaron and yujin on the TV in a prison uniform. There's also a bit when yujin is sleeping where he says "sorry dad" or smth like that. Yujin also often hints at having a good life before something happened like how Santa "stopped coming".

I think Yujins dad kidnapped Aaron for money or something since he probably couldn't afford Christmas presents and Yujin helped Aaron escape. He then got his dad arrested but doesn't know that Aaron was the one he helped. However, Aaron knows which is why he gets comforted by his voice. Maybe it's because Yujin would sing to him before he helped him escape? This could also be why Aaron knew Yujin wanted to be a singer when he was younger. This is also just a theory btw