Emma Cullen appears to be perfectly, purely human. And she would be… if her great-grandfather hadn’t made a deal with the Fae in order to make his way to America. This resulted in a grandmother who was a Changeling, and one who married and passed her Fae blood to her son who then passed it to his daughter.
She is only a quarter-Fae, just enough for monsters to cotton on that she isn’t completely human. And she may still have enough monster in her blood that she will be taken to Faery when it comes time for her to die, like her grandmother and father before her.
But her Matthew was human… and she does indeed seek revenge.