Gerald Mallory is one of the first vampire hunters who is still alive. This is one of the reasons why some consider him to be the head vampire hunter, although by the time of Skin Trade he doesn't have the highest kill count in either vampires (Anita) or rogue lycanthropes (Denis-Luc St. John) despite being in the business for at least ten years longer than Anita.
Mallory has such blinding, irrational hate towards vampires that Anita suspects it makes him less effective, and causes him to miss clues. He's also a stake-and-hammer man who refuses to use guns and silver ammo. Mallory thinks Anita is the whore of Babylon for sleeping with vampires, and in Hit List we hear that he uses her as a proof that women are more likely to be seduced by vampires than men on his guest lectures for new marshals of the preternatural branch. According to Mallory, Anita was one of the best vampire executioners before she gave in to Jean-Claude, but isn't anymore.
Politically Mallory has a lot of clout, and in Skin Trade we learn that he has pushed through a special weapons bill for the federal marshals of the preternatural branch, which allows Anita to carry old fashioned phosphorus grenades with her, even though they are still illegal against anything else than the undead and the shapeshifters.