Well, as far as I'm aware, GM may correct me, but your actual physical appearance is a reflection of your soul. We so far have been rather uncreative in that field, but hypothetically you can look however you want. I've not the slightest clue on playable demons though. Perhaps a creative half-demon. Like a soul that is damaged, and an equally damaged demon latches on to create a whole that won't dust away(soul dying). That's just a theory, I've no idea if the GM would pass that one.