Anastacia is the Fire Keeper of Firelink Shrine and is bound to the bonfire to keep it lit.
Initially, she is unable to speak due to her lack of a tongue. If Lautrec is allowed to kill her after both Bells of Awakening have been rung, the player can later invade him in Anor Londo using the Black Eye Orb found on her corpse and retrieve her soul and use it to restore her to life. Her tongue will also be restored, allowing the player to engage in conversation with her.
Anastacia is mentioned to be "impure", however, what exactly made her so is never revealed. It has been hinted from item descriptions that Anastacia chose her own banishment and even imprisonment.
Her Dingy Set implies that she was maimed to prevent escape.