Called "The Fair Lady" by her attendant Eingyi, she is the leader of the Chaos Servant covenant. Blind, weak, and on the verge of death after swallowing the blight pus of the residents of Blighttown, she is completely immobile and was made Fire Keeper of the nearby bonfire to keep her alive. Her speech seems incoherent at first but with the Old Witch's Ring equipped, the player may talk to her normally. She mistakes the player for her sister, Chaos Witch Quelaag, as she is the only one who visits (and can talk) to her.
This unnamed Daughter of Chaos once lived with her mother - the Witch of Izalith - and her siblings in the ancient city of Izalith. After the Flame of Chaos incident, most of the sisters were transformed into nefarious demons. She and her sister Quelaag escaped the worst of it, having their lower torsos bound to giant spiders. Fleeing their deranged, demonic kin, they climbed to the base of Blighttown and fashioned themselves a home.
The precise details of what came next are only suggested at and implied by Eingyi. The Daughters of Chaos brought denizens of Blighttown with them as servants. Quelaag was unmoved by their suffering, but the Fair Lady wept for them. Overwhelmed with empathy, she did the only thing she could to help: she swallowed their blight pus, despite orders against such action from Quelaag. This cured the residents of Blighttown, but left the Fair Lady in her current state - near death, surrounded by eggs that will never bring life, and now, due to the actions of the player, completely and utterly without help.
The Fire Keeper Soul she possesses states, a Fire Keeper's soul is a draw for humanity, and held within their bosoms, below just a thin layer of skin, are swarms of humanity that writhe and squirm. To her, the countless eggs which appeared were cradles for each tiny humanity.