Jack was never a doctor, but "studied surgery" by cutting people open to see what made them tick. Jack claims even when he was human, he wasn't really human. He eventually kidnapped, murdered, and extracted the organs of 4 women. Wyatt and Doc pursued him until Doc got trapped in a well for 130 years. At some point Wyatt killed Jack with Peacemaker and turned him into a revenant. As part of The Seven, he is partially responsible for the deaths of Josiah and Ward Earp.
The Jack of Knives only appears in the episode Two-Faced Jack. Jack got his assistant to kidnap Wynonna in the previous episode. Once Wynonna realizes Reggie isn't the revenant, Jack kills him, saying he's "served his purpose". Wynonna correctly guesses that he needed a human to handle Peacemaker. Jack taunts Wynonna by talking about some of the past Earp heirs and about Ward Earp's deal with Bobo Del Rey. Doc and Dolls arrive to save Wynonna and give her the opportunity to shoot Jack with Peacemaker.