What I think it is it's another senerio where like when the whole corpse and vampire those were all senerios. Basically what this part of the story isn't happening in the real world their like in another dimension. Idk about the whole demon thing other than the fact that the white haired dude was sick and almost died; then which I think made this deal where he would have to get his soul mate to love him through the lifetimes (I think). To which we meet are main character. I think if not we'll idk but I'm like 99 percent sure of myself. If this is not the case it is still a good explanation.