
It’s pretty much him, his night self it's more like a version of his subconscious that comes to the surface releasing all his primary emotions that he tends to repress since young. The curse helps make everything even more extreme making him go insane little by little
So I don't see it as necessarily a split personality, it's more of an extension of himself that he neglects but is forced to come out in a really extreme way because of the curse

From what I could understand in the story, the idea of the curse was initially to kill him quickly by completely opening the gates to the underworld version. Not only would he die quickly due to the pressure that this would create, being both physically painful and mentally would driving him crazy because of the demons. Since he wasn't bitten, the curse was half-hearted, only partially opening the door, making the process much slower but bearable thanks to the sage (pls correct me guys if I'm wrong)
Has it been said yet what the curse is. As in is he still him at night or is a completely different