Vivien (sometimes called Nineve, Nimue, Niniane). Vivien is the woman who entombs Merlin in a cave or a tree. Although Merlin can foresee his own fate, he is unable to prevent himself being captivated and and then walled up by Vivien.
In Malory, Nyneve, is one of the Ladies of the Lake. She deprives Arthur of Merlin, but she rescues him twice, first by saving him from Accolon who has been given Excalibur by Morgan le Fay to use to kill Arthur, and after that, by stopping him from wearing the destructive cloak sent to him by Morgan.
In the French Vulgate Estoire de Merlin, Vivien woos Merlin and imprisons him in a magical tower so that she can keep him for herself. Vivien visits him regularly and they make love together. However in the continuation of the story, the Suite du Merlin, the relationship is very different. Merlin is sealed in a magic tomb and dies a slow death.
Tennyson turns Vivien into the epitome of evil.
Full references to more about Vivien at the Rochester site
Characters from King Arthur's legends