Mercy Sparx is the story of a devil-girl secretly hired by Heaven to hunt down rogue angels hiding amongst us. She doesn't live in a dark hellish cave or an ominous fortress or have many resources. She lives in a crappy apartment, and is usually drinking when not hunting down angels. To see her on the street you wouldn't know she was any different than you or I, but with a thought her otherworldly abilities (and appearance) boil to the surface.
She doesn't want this responsibility, but she's been forced into executing the job, and quitting is not an option.