So I think basically he asked to be gone and not to be there so he turned him into the small soul crystal so that he's not there but still...there ya get me
I see.
As a fellow reader myself, from my point of view, the white hair dude is still alive until the crystal were destroyed.
He can't remember the fragments of the hyung no more, so he found no purpose in life and asked to be killed.
However the Mr Park had already fallen in love with him, so instead of killing the kid, the demon took his soul as the contract of subordination (which was stated somewhere in the panel on last extra chapter).
Mariam also took Beokhee's soul, but that's a different case there since they love each other.
But for Seolwoo and the other demon, it's not the same. Seolwoo wanted to die, but the demon keep him alive by taking his soul. That's why Mariam called the Mr Park an idiot, because he formed a subordinate contract with the human who both does not have the will to live and love him.
So in the end, did that guy die or is he alive until that soul crystal thing is eaten/destroyed? I skip to the extra but it sounds so open ended that I don't know if I want to see the whole story or not.
Edit: I am very unhappy with the open ended ending. Like this could have been so much more but it feels like the author just don't want to dwell on this much and wanted a happy ending for the main characters. I want a proper ending for the side characters and how they navigate their life because it makes it seems like the cop died meaningless for the killer. And now Mr Park seems to be in a crossroad situation.