[@Zapdos] The Templar idea forms stronger and stronger in my mind already BUT, I've got yet another idea that might prove quite interesting and that I've been meaning to play for a while. 

A vampire but not in the traditional sense. In a CYOA book I read vampire were people who's souls have been replaced by a demon. So, one soul goes on, other soul is destroyed but not in all cases. In very rare cases, a person's soul was very strong (lots of willpower) and instead of being replaced, they fused together. 2 souls, 1 body. They didn't drink blood but drank on the soul of the creatures. 

The character I'm thinking will be a girl, young and innocent while her demon is the opposite but they co-exist. Each with skills that are opposites to one another. Two characters in one, basically. How does that sound? If it's a no-no, then I'll go for the templar.