Earlier sources show Morgan le Fay as a healer, but with the passage of time she evolves into a more witchlike character, scheming to get rid of Arthur and his companions and using magic to those ends. Morgan le Fay rarely appears in post-medieval works, until the twentieth century when there is a renewed writing on her character.
Geoffrey of Monmouth tells us that Morgan Le Fay was the leader of the nine maidens of Avalon, who took Arthur away in a barge after the Battle of Camlann. Morgan is said to be the first of nine sisters who rule The Fortunate Isle and is shown to be a healer and that she could fly and change shape. She says that she can heal Arthur's mortal wound provided that he stays with her for a long time. Vita Merlini (c. 1150)
Chretien de Troyes continues with the idea of Morgan Le Fay as healer. Chretien also tells us that Morgan le Fay is Arthur's sister, her mother being Igraine and her father being Gorlois.
The Vulgate Cycle shows a Morgan le Fay who is still friendly towards Arthur but bitter towards Guinevere who broke up one of Morgan's own romances. Morgan le Fay tries to seduce Lancelot and also to expose his affair with Guinevere. In the Prose Tristan, she brings a magic drinking horn to Arthur's court from which no unfaithful lady can drink without spilling.
After the Vulgate Cycle, writers such as Malory show a Morgan le Fay angry also with Arthur (because he had killed one of her lovers). Using both magic and guile, she plots Arthur's downfall. For example when Accolon gets the chance to kill Arthur with Excalibur. Malory does however show her as a mortal, having to use magic to try to stay looking young. Morgan le Fay is cited in Morte d'Arthur to be Arthur's half sister, the daughter of Arthur's mother Igraine and her first husband, the Duke of Cornwall.
Full references to more about Morgan le Fay at the Rochester site