Gwynevere is the only daughter of Lord Gwyn, making her the Princess of Sunlight (she calls herself Queen, but it's actually mistranslated from the original Japanese). She appears as a giant woman resting in her chamber in Anor Londo but it is actually an illusion, created by her brother, Gwyndolin.
The real Gwynevere is currently not in Lordran and is married to the Flame God Flann. If the illusion is attacked and destroyed, the city will turn dark and certain other NPCs will become hostile. This also reveals that most enemies in the city are also illusions.
Gwynevere is meant to appear as a comforting, motherly figure. She appears as a fair lady with brown hair and a kind face. The light shines brightly from behind Gwynevere's figure, and she is dressed in fine white cloth with golden bracelets and jewelry.