Donald "Donnie" Addie Pfaster (played by Nick Chinlund) was a serial killer from Minneapolis. He initially seemed by all outward appearances an extraordinarily ordinary man, however, it soon became apparent that Donnie was a recluse and a "death fetishist".
He liked to collect the hair and nails from dead bodies, particularly from women. He worked at a funeral home, and was fired when his fetish was discovered. Eventually, Donnie began to yearn for living prey, and started murdering women like the woman he brought home from the streets of Minneapolis.
First Encounter with Dana ScullyMulder and Scully became involved in the investigation, which proved to be very difficult for Scully due to her recent brush with death. Things became worse when Pfaster targeted Scully herself. Pfaster held Scully captive and defenseless as he prepares to perform his sadistic ritual. Once distracted, Scully manages to escape from her bonds and Pfaster chases her around the house while armed with Scully's firearm. When Pfaster was about to kill her, Mulder and the FBI saved her and Pfaster was sent to prison in Illinois. (TXF: "Irresistible")
Second Encounter with Dana Scully
In 2000, Pfaster was deliberately released from prison by Robert Gailen Orison, who thought it was his divine duty to release prisoners sentenced to life so that he could kill them himself. However, Pfaster quickly escaped Orison's control and resumed murdering women, including a woman called Blueberry. Orison caught up with Pfaster, but was killed by him when Pfaster revealed that he was actually a demon in disguise.
Pfaster made his way to Washington, D.C. where he found Scully in her apartment and attacked her. He called her "the one that got away" and had been obsessing about her ever since. Mulder arrived at her apartment in time to save her, but Scully shot Pfaster dead before there was a chance to arrest him. The incident was very traumatic for her, but she believed that what she had done was right. (TXF: "Orison")
At times, Pfaster's appearance was seen to shift visually, usually to something resembling a demon. When Scully was abducted by Pfaster, she witnessed him change appearance several times, including several human guises as well as his demonic form. In interviews, Chris Carter remarked that this was influenced by victims of serial killers recounting how their captors seemed to change shape during their encounters. Whether or not this shape shifting is reflective of Pfaster's true demonic nature is open to debate.