Jimmy

Has the appearance of a young boy with glasses and dark hair, but is really the favorite servant of Ulysses, being a member of the Unseelie Court. He is a spectral hound, and can shapeshift at will. Before Lydia and the others knew what he was, he spied on them by acting as a messenger and friend of Slade and the Scarlet Moon organization, under Edgar.

(Source: Earl and Fairy Wiki)