Jim Miller, known as "Killer Miller" and as the "Shadow Assassin" was a forhire killer in the late 1800's. His first kills were his grandparents and his brother-in-law for unknown reasons. He got his nickname by "moving like a shadow". It's speculated by Doc Holliday and Levi it was Miller who killed Jesse James, not Robert Ford. At some point, Miller was arrest and killed by Wyatt Earp.
As a revenant, Miller has the ability to move through shadows and absorb light. He also wears an old school bullet-proof vest made of metal.
Miller is only seen in the episode Keep the Home Fires Burning. A security guard wanders in the warehouse he dwells in and accidentily cuts himself. Miller accepts the guard's blood as payment, but when he wont name a target, Miller kills the guard instread. Later in the episode, Bobo Del Rey gets the idea to use Miller to kill Wynonna Earp so that he can steal Peacemaker. He inlists Doc Holliday's help since he needs the blood of a mortal as payment for Miller. Doc didn't know he needed something of the target to give to Miller, so he gives him the only thing of the Earps he has, Waverly Earp's revenant journal. Miller attacks the Earp sisters at the Earp Homestead that night. With the help of Xavier Dolls, the girls are able to send Miller back to hell and remove the talisman that allowed revenants on their land.