[@Zapdos] The whole "I'm a pure innocent woman that belives killing is bad" vs "I'm a blood-thirsty monster that has seen billions killed, tortured in the worst ways possible" spiel also stroke a cord in me. Two personalities that are at odds with one another. Two people that are wholy different. From a story perspective, they'll both come to terms to whom they are now. They'll grow into a strong pair. They will start discovering that they're not as different as they think they are. 

Yes, at the start of the story, they would be average. Maybe even weak (the human part at least) but that's the point, right? Have characters that are novices/a bit above that grow into real heroes or so. 

Blood magic - control of a person's blood in different ways. Stopping it to flow for a few sec, forming blood into solid forms and flinging it at targets.