Vivianne.Another thing I think would be really interesting is a demon companion (I personally would prefer a dude; the party's kind of a cheesecake factory) who's part of the party because they know you're out to stop Kas and they want to help with that. Basically someone who wants to be a sort of positive example of demonkind since Kas and her flunkies are out there giving them all a bad rep, and they want to prove that there's no such thing as a people who are inherently bad. Maybe they were duped into following her through the portal because Kas fooled a chunk of demons into thinking they'd be settling whatever land's on the other side, rather than conquering.
The main issue I have is that, really a person is supposed to get corrupted or have a certain attitude and personality (or be driven to it) to the point where they lose their soul. They can keep their personality and such, but having them show altruism and compassion and such except in narrow and specific instance or towards specific people lessens the horror and impact of demonification (in my opinion).
Obviously I am not saying there's anything wrong with wanting something like that (and posting it here, which is what it's for), I just hope it doesn't become that. Maybe someone that's tainted or corrupt a bit, but once they're a demon, it's because they reached a point where they gave into what being a demon means (even if that's still has a broad range).