Jim Miller, known as "Killer Miller" and as the "Shadow Assassin", was a for hire 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 that it was Miller who killed Jesse James, not Robert Ford. At some point, Miller was arrested and killed by Wyatt Earp.
Miller is only seen in the episode Keep the Home Fires Burning. A security guard wanders in the warehouse that Miller dwells in and accidentally cuts himself. Miller accepts the guard's blood as payment, but when he wont name a target, Miller kills the guard instead. Later in the episode, Bobo Del Rey gets the idea to use Miller to kill Wynonna Earp so that he can steal Peacemaker. He enlists 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 on him, Waverly Earp's revenant journal. Miller attacks the Earp sisters at the Earp Homestead that night. With the help of Xavier Dolls, the sisters are able to send Miller back to hell and remove the talisman that allowed revenants on their land.
- Shadow Teleportation: Due to his ability to move amoung shadows as a human and/or his nickname "Shadow Assassin", Miller has learned how to travel via shadows.
- Energy Absorbtion: Miller is able to take the energy from sources such as lights, fires, and batteries.
- Telekensis: Like some of the revenants, Miller had a certain degree of telekinesis. In his “den” he was able to close doors without touching them and was able to partially drag Henry into his cavern.
- Assassin: Killer Miller can be hired at the price of the blood of a mortal. He must be given an object of his intended target to be assigned a victim
- Tracker: Miller is able to track his victims using only one of their personal possessions. He was also able to tell the notebook given to him by Doc had belonged to Waverly rather than Wynonna.
He also wears an old school bullet-proof vest made of metal.