Adventures of Beatrice by H. C. Herman

Beatrice is only half human, but she doesn't know it. As far as she's concerned, she's just another orphan stuck in the foster system, constantly getting into trouble with her evil Foster Monster. Beatrice is in the middle of escaping captivity when she accidentally finds a burnt letter in the fireplace. Only a few words survived the flames, but they're just enough for Beatrice to realize she might not know herself as well as she thought.