Sister Mary Eunice

Sister Mary Eunice McKee (portrayed by Lily Rabe) is a young nun originated from American Horror Story: Asylum who works at Briarcliff under Sister Jude who favors her, believing her naivety to be an indication of her "purity".

American Horror Story
Sister Mary Eunice was a plesant woman and took care of Pepper when she was (falsefully) accused of murder. She seeks to please everyone, which proves to be a difficult task in the Briarcliff setting. Mary Eunice puts much trust in Dr. Arden, who regularly asks her to deliver buckets of meat to the creatures in the woods, which Arden tells her little about. However, after Jed's exorcism she becomes possessed by the Devil, and her innocent personality quickly disintegrates as the spirit takes control of her, causing her to become sly and manipulative. Mary Eunice kills a patient, feeding her corpse to Arden's creatures.

Under the possession, Mary Eunice begins to formulate a plan to gain control of the Vatican. When Jude is removed as head of the asylum, Mary Eunice quickly takes her place. When speaking to the Shachath, the real Mary Eunice manages to break free from the Devil's reign and begs for death, before the Devil quickly regains control and silences her. After Jude hires a Nazi hunter to find out whether Dr. Arden is a former Nazi scientist, she kills the inspector and keeps some of the evidence to blackmail Arden with. Jude, realizing that Mary Eunice has been possessed, intends to kill her, before Mary Eunice locks her in a cell with Leigh Emerson. She frames Jude for the murder of Frank, and admits her into the asylum as a result.

Knowing that he is "Bloody Face", Mary Eunice hires Dr. Thredson as a full-time psychologist at the asylum. After Monsignor Timothy is crucified by Leigh, he is visited by the Shachath who warns him that the Devil is living at the asylum in one of the nuns. Mary Eunice nurses the Monsignor back to health, and he attempts an exorcism, which does not work. She then rapes him, before trying to convince him to work with her to gain control of the Vatican, with him as Pope, to which he declines. Mary Eunice corners the Monsignor on the stairway, before the real, innocent Mary Eunice breaks through from the Devil's reign and releases him, begging for death. The Monsignor immediately pushes her off the stairway, before the Devil can regain control of her body.

The visiting Shachath, Angel of Death comes to claim both her and the evil spirit.

LOTM: Witnesses of Sleepy Hollow
After death, Sister Mary Eunice had become a saved soul who can walk pass realms and guide people who lost their way. She later joined Team Witness to defy the evil once more.