Lore
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 though it is strongly suggested by her comments on her impure tongue that it is due to something she has said.
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.
Cut Content
Theories
How Anastacia ended up at Firelink Shrine
Dialogue
Sources
NEXT: Andre of Astora
"It is thought to have once been the white hood of a maiden" should actually translate to “the white hood of a saintess”. The same title "saintess" (聖女) is also used on Leah.
Which means, she is likely to used to possess some religious status within the Way of White. The cut dialogue "Vereor Nox", which means "May the fire guide you" is also a hint of Way of White background. The "impure" dialogue is also likely to refer to a religious status, which serves to justify using her as a sacrificial victim.
She could be a man-made undead like Leeroy, Leah and other Way of White undeads, and is placed here intentionally to serve a purpose (robbing her of her firekeeper soul is a sin that attracts divine attention, which leads to the black eyeball allowing you to invade Lautreac).