It's not clear if Mr. Sanderson can be considered a character or more. There seem to always be more than one of them. He (or they) serve as the Head Pixie's goons. There is usually a "main" Sanderson that stands alongside Head Pixie most of the time and he, along with H.P., deliver most of the dialogue from the Pixies.
Unlike the fairies, he is square shaped, including his wings. His pointy hat and business suit are all gray, except for certain situations in where he changes his clothes. He looks and speaks exactly like all the other Pixies with the exception of his skin tone (which is much tanner than the other Pixies) and his cowlick which occasionally is slicked back time to time.