The basic hook for the series is that the Vatican maintains a secret army of warrior nuns and priests that hunt rogue Satanists, demons, and other manifestations of evil on Earth. Mary joined that group some eight years before after studying warrior tactics with her more conventional order. Having survived so long, she's now one of the more experienced devil hunters around, employing earthly weaponry as well as faith to stop Evil's spread.

Shotgun Mary - Deviltown #1