Regan MacNeil is a fictional character from the horror novel and film series, The Exorcist. She is portrayed as a young girl who becomes possessed by a demonic entity, leading to terrifying changes in her behavior and personality. Her struggle with this possession is central to the story, highlighting themes of good versus evil. The character is known for her iconic scenes, particularly the dramatic exorcism performed by Father Merrin and Father Karras. Regan's transformation and the impact of the possession on her family and those around her have made her a significant figure in horror literature and cinema.