Also known as the Lady of the Darkling, she is the Fire Keeper in Anor Londo. She wears the Brass Set armor to hide her hollowed form and help her hunt the guilty as a Darkmoon Blade. Her wielding a Parrying Dagger may suggest that she is originally from Carim.
She also acts as a guide for the city of Anor Londo, giving directions to the Chosen Undead.
If the Chosen Undead destroys the illusion of Princess Gwynevere, The Lady of the Darkling will turn hostile and wait to challenge the Undead at the cathedral side of the spinning bridge. Likewise, she will become hostile if Gwyndolin himself is killed.