Kate is a vampire, and the mate of Luther.
Background
Kate was responsible for the murder of Daniel Elkins, and the theft of the Colt.
She is later captured by the Winchesters and is to be traded to Luther for the Colt. She manages to break free only to see her mate, Luther, shot and killed. She escapes with another female vampire whom she had earlier turned. They are the only survivors as the rest of the vampires are taken out by Sam and Dean Winchester.