Another fanfic idea:
The Sylph of Nephelrune
In the town of Nephelrune, there has been sightings of a strange monster-woman stalking the streets at night, with glowing white skin and asymmetric horns, with a pair of fangs sticking out from her jade-painted lips. However, with her appearance, once a week a woman in the town wakes up woozy with a pair of bitemarks on her neck. While they are fine afterwards and ultimately little harm has been done, it has become increasingly clear that this being is drinking these women's blood, and henceforth a bounty has been declared upon her.
Meanwhile, in the sewers beneath, this same figure cuts down the monsters beneath the city streets with terrible speed and a roaring blade, while providing a guiding light to those lost in the dark. Not only that, but she has managed to cut off the Hand's access to the sewers and the streets by scaring off dealers, causing them to target her as well.
However, despite having the ire of both the government and the Hand, she has not taken action against either, content to deal with whoever gets sent her way...