Six is Kin, a deeply rare Fey-Darkness crossover, and a trained assassin. She is one of the Darkling Twelve who serve under the Raven King
See Kanon.. Influential in Notte’s life, she taught Notte
He’s one of the oldest living beings among the Mythos. The... self-defense and assassination during his time in the city of Az’Kabek.
Also, incidentally, Six is dedicated to a wonderful reader: Sixelona.