Fray by Joss Whedon

Set in a gritty, dystopian future, the story follows Melaka Fray, a young street thief who discovers she is the latest in a line of demon-hunters called Slayers. Pulled into a hidden war against vampires and other supernatural threats in her city, she must confront a violent past and the painful transformation of her twin brother into one of the creatures she is destined to destroy, forcing her to learn to fight, trust unlikely allies, and decide what kind of protector she will become.