Erm, angels and demons are people that have died(there's no eternal waiting room where the souls wait for the world to end). Hell isn't horrible unending fire and heaven isn't unending bliss. Angels have tasks assigned to them, so far I know two kinds. To go to the mortal world using a shell, a sort of temporary body, on a task to do a kind deed on one specific person. Or to go to the mortal world without a body and pick up the soul of a dying person. Demons have something similar, just with bad deeds and collecting the souls of bad people(I still need to find out how they classify them...).
Arya's an angel, Nathan's an angel and Nico's a demon. Arya and Nico fall in love through various encounters in the mortal world, and obviously that's not allowed. I'm not sure what Nathan has to do with it, he just keeps throwing ~~mysterious~~ hints of his past with Arya to me -_-
I know the ending, but not much about what happens between the beginning and end xD I like the god person, but they are hard to explain. Sort of like the force of nature, I guess? Or the ever changing reflection of humanity.