In Malory, she is the companion to a previous lady and takes over in that role. Also in Malory, she is the one who bestows Excalibur upon Arthur and receives it back from him in the end. She is in the barge that bears Arthur to Avalon. She also travels with Merlin, who has fallen in love with her, through Brittanny and Cornwall, learning his arts but never consenting to be his mistress.
Other works have her as the woman responsible for Lancelot's upbringing. In Malory, the Vulgate Cycle and other settings of the tale, she casts a spell over Merlin with the very magic he has taught her. Some texts have her kill him but usually Merlin is imprisoned in a cave or a tomb, never to return to court. Later she uses magic in a less evil way to separate Pelleas and Ettard so that Pelleas can become her lover and eventually her husband.