Quelana of Izalith is one of the seven daughters of the Witch of Izalith, currently staying in Lower Blighttown. When her mother attempted to recreate the First Flame, failure appeared in the form of what it's now known as the Chaos Flame. The flames of Chaos engulfed Izalith, killing or mutating the majority of the population into demons. Quelana was able to escape without suffering mutations, unlike her sisters (the Fair Lady and Quelaag) or her brother.
Over 200 years before the events of Dark Souls 1, Quelana took in Salaman as her pupil. Being as bungling as the Chosen Undead, Quelana taught Salaman in the arts of pyromancy. It is believed that Salaman went back to the Great Swamp, where he was called 'Master Pyromancer'. He's believed to have started the teaching of pyromancies in his homeland, teaching people like Carmina and possibly Laurentius.
For her teachings, Quelana is called to be the 'Mother of Pyromancy'.
Quelana seems to have a notable survivor's guilt. She asks the Chosen Undead to put her family, specially her mother, out of their misery. Eingyi refers to Quelana as the "Lost Quelana" and mentions rumors of her wandering in the poison swamp. He suggests that she could be another Daughter of Chaos, and implies that the Fair Lady would be comforted by her presence.
She wears the gold hemmed black robes, a set of clothes that are said to have been wore even before the Age of Fire. Dark Souls' intro cutscene supports this, since the seven daughters - and their mother, too - are shown to be wearing these clothes.