Board Thread:Dark Souls II/@comment-2.223.169.191-20140325204725/@comment-16047389-20140818204710

It's more likely the Witch of of Izalith's Soul reincarnate. If you follow the story along (including the DLC now), it is made apparent that the four Great Souls that you collect in Dark Souls 2 have in fact reemerged numerous times throughout history. The four Great Souls are that of Gwyn, Nito, the Witch of Izalith, and now Seathe. I believe that Seathe managed to improve his own soul to be on par with the others by his use of his crystal research, as well as using the piece of Gwyn's soul that he was gifted. But that's a theory for another thread.

The point is, the four souls seem to reappear everytime the First Flame begins to once again wane (likely for the purpose of being recollected by another Chosen Undead, and used for kindling once more). While the Lost Sinner's body may not be that of the Witch of Izalith herself, the soul is most certainly hers. For further credence of this, during NG+ in the fight against the Lost Sinner, you are attacked not just by the Lost Sinner herself, but two Pyromaners. And as we know, the Witch of Izalith was turned into the Bed of Chaos along with two of her daughters. This extra touch to the fight wouldn't be as significant if it was Quelana.