Hmm, a new Sin Eater perhaps? Actually... she's wearing black and gold and has something vaguely mask-like... an Amaurotine?
Spoilers from MSQ:
She's a summoned hero / Warrior of Light from another shard. From the little glimpse at her personality we know she's basically a psycho necromancer, who feels she's tainted with her powers, but she also enjoy using them quite a lot. We do not know if she is personally chosen by Hydaelyn or she became one thanks to Elidibus machination or she is one of your fragments.
Either way, I find the existance of an insane necromancer hero quite hilarious. We've also seen a berserk hero at the end of the dungeon. Crazy stuff.