Denis-Luc St. John is a vampire hunter and a federal marshal. To Anita's knowledge he's unmarried.
In Incubus Dreams , when Vittorio comes to St. Louis, he's already been at New Orleans (among other places) and left five bodies behind there, as well as almost killing Denis-Luc himself. He's still in intensive care a month later when the St. Louis police connect the dots, and still in hospital at the end of the book although by then well enough to be reached by phone. He makes Anita promise to call him if Vittorio ever resurfaces.
In Skin Trade when Vittorio does resurface Anita tries to call Denis-Luc, but catches his sister instead, who doesn't pass on Anita's message. Denis-Luc is upset when he hears, but Anita can't blame the sister for wanting to keep her brother alive, since their previous encounter landed him in the hospital for months and then rehab on top of that.
In Dead Ice we learn that Denis-Luc raises troll-hounds. He has the highest kill count of rogue lycanthropes within the United States, at least in part thanks to his dogs that let him track supernatural creatures through wilderness where other breeds of dogs usually refuse to do so.