According to Geoffrey of Monmouth, daughter of Uther and Igerne (Igraine), Arthur's full sister and the wife either of King Lot of Lodonesia or of King Budic of Brittany (Budicius), and mother of Gawaine and Mordred.
De Ortu Waluuanii relates how Anna fell in love with Lot, who was staying at Uther's court as a hostage from Norway. They engaged in a clandestine affair, which produced the illegitimate Gawaine. Anna, who had kept her pregnancy secret, decided to send Gawaine away at his birth to avoid trouble with Uther - but she sent with him a ring and parchment attesting to his lineage, which he later used to gain entry to Arthur's service.
In his play The Misfortunes of Arthur, Thomas Hughes uses Anna rather than Morgause, but he makes Mordred a product of Anna and Arthur's incest, which Geoffrey does not.
L.A. Paton quotes a source which says she was also called Ermine and that she married Budic while her sister married Lot. Miss Paton wonders if she is to be identified with Morgan. The possibility that she was derived from the Celtic goddess Anu, the earth mother, cannot be ruled out.
In a note to 1. 8135 of Chrétien's Perceval, D.D.R. Owen indicates Anna as presumably Ygerne's (Igraine) daughter and the second of the queens of the Rock of Canguin. (Cline however, in a note to 1. 8135 of her translation, follows Hilka in mentioning Morcads (Morchades), which is much closer to Morgause, as the name of Arthur's sister and Lot's wife.)
Anu | The Legend of King Arthur
Beuno | The Legend of King Arthur