The Voodoo Lady appears in all five games as Guybrush's guide. The proprietor of the voodoo shop chain International House of Mojo, she is a voodoo priestess. She is voiced by Leilani Jones Wilmore in all of the games except for Tales of Monkey Island, where Alison Ewing had the role instead. The Voodoo Lady's role in The Secret of Monkey Island is to provide Guybrush with a rubber chicken with a pulley in the middle on Mêlée Island and to predict Guybrush's future, which involves sailing to Monkey Island, entering a giant monkey, and either being helped or eaten by the Monkey Island cannibals. In Monkey Island 2: LeChuck's Revenge, she opens a voodoo shop in a swamp on Scabb Island. In addition to providing much helpful information, she creates a voodoo doll of Largo LaGrande and gives Guybrush a book on the legend of Big Whoop.
By The Curse of Monkey Island, the Voodoo Lady has moved to Plunder Island, setting up another shop in a ship half-sunk in a swamp and spends time playing with "the Voodoo Kids". The Voodoo Lady helps explain some of the plot between the second and third games, and tells Guybrush how to lift a curse placed on Elaine by a cursed diamond engagement ring, directing him to Blood Island to do so. In Escape from Monkey Island, she returns to Mêlée Island. She informs Guybrush about the "Ultimate Insult", a powerful and dangerous voodoo talisman, and helps him find its components. In that game it is revealed that the Voodoo Lady uses her business to sell voodoo products which, do to the issues in that game, she is low on. The Voodoo Lady emerges again in Tales of Monkey Island, in which she is an informant for a local newspaper on Flotsam Island, using her powers to get stories from the other islands, as Flotsam's winds stop ships from leaving. She tells Guybrush how to cure the voodoo pox that has afflicted his hand. In the penultimate chapter of the ToMI storyline LeChuck exhibits a journal, proving that the Voodoo Lady has masterminded every single event in the Caribbean since The Secret of Monkey Island, thus contributing to unleash LeChuck's menace on the world. Voodoo Lady doesn't deny this, hinting at deeper motivations beneath her seemingly malevolent actions. Guybrush and Elaine lose their trust in the Voodoo Lady and after killing the again-evil LeChuck, they believe they are free from the endless loop of killing LeChuck. However, the Voodoo Lady pays dead pirate hunter Morgan LeFlay for giving her LeChuck's remains by returning her the mortal world. It is suggested that the Voodoo Lady plans to continue the cycle.
Both Guybrush and LeChuck have noted that no one knows the Voodoo Lady's name. When confronted, she states that it is not important, although later she admits that names have power. Guybrush asks if his name has power, which she answers with "Sure, Why not?"
In Tales of Monkey Island, it is revealed that the Voodoo Lady once had a boyfriend named Coronado De Cava who was a world explorer, who searched for La Esponja Grande on her behalf.
Voice acting have been done by various actresses over time:
Leilani Jones in: The Curse of Monkey Island (1997), Escape from Monkey Island (2000), The Secret of Monkey Island (Special Edition from 2009) and Monkey Island 2: LeChuck's Revenge (Special Edition from 2009)
Alison Ewing Tales of Monkey Island (all chapters, 2009)
Source: Wikipedia, "World of Monkey Island," available under the CC-BY-SA License.