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 they, along with H.P., deliver most of the dialogue from the Pixies.
Unlike the fairies, they are square-shaped, including their wings. Their pointy hat and business suit are all gray, except for certain situations where they change their clothes. They look and speak exactly like all the other Pixies with the exception of their skin tone (which is much tanner than the other Pixies) and their cowlick which occasionally is slicked back from time to time.
They first appear in "Pixies Inc" as a side character when Timmy Turner finds out that Fairy World has been bought out by the pixies in a hostile takeover. All of the Pixies seen are copies of Mr. Sanderson, with the exception of Head Pixie. Timmy had been working on a miniature golf course for his science project but had the wish rejected because of it. Mr. Sanderson introduces themself to the trio when Timmy makes a complaint. They end up going over Cosmo's portfolio, and realizing he is an easily manipulated fool, the Pixies decide to make him "Rice President". When Timmy challenged Head Pixie to a golfing competition, Mr. Sanderson attempts to sabotage Timmy's effort to win by using magic to manipulate the golf balls. Cosmo, however, countered back when he realizes that the people he loves are more important. HP nearly wins, but thanks to Jorgen bouncing on a pogo stick, Timmy wins. As per the deal, Timmy was allowed one rule free wish which was that the Fairies owned Fairy World again. After that Jorgen is shown using H.P., Mr. Sanderson, and the other Pixies as darts for a dartboard.
In "Wish Fixers", Mr. Sanderson controls a robot copy of Jorgen Von Strangle from the inside as a way of tricking Timmy and his fairy godparents, by making them undergo a "shock therapy" program headed by Head Pixie (Timmy was hesitant at first, but Mr. Sanderson in the Jorgen robot threatened him to sign it or else he will use his bones as toothpicks). When Timmy figured out what was going on, he wished for the real Jorgen Von Strangle to appear, who ripped the robot version in half to expose Mr. Sanderson manipulating it from the inside. As punishment, H.P. and Mr. Sanderson were reduced to the size of bacteria and were forced to be around some amoebas which they noted to be more boring than they are. Mr. Sanderson even noted the single-celled irony in all of this before they and H.P. both said "Aaaah!" in a boring but dramatic tone. Timmy, his godparents, and Jorgen all laugh away at this after examining it through the microscope.
Many years ago, Mr. Sanderson and H.P. took an infant Flappy Bob away from his family to use him as a vital part of their complicated 37-year plan to make kids and adults go against each other, so the kids would wish to rule everything, and no longer needing the fairies, have Flappy Bob make a deal with the Pixies to have the world run by him where meanwhile they can take over Fairy World since fairies were no longer needed by children due to the wish. However, later Flappy Bob realized that they were using him all along and soon used a loophole in the contract to return everything to normal. Thwarted, they were forced to return Earth and Fairy World back to normal before the Fairies made H.P. and Mr. Sanderson pedal all the way back home. HP notes they should try doing a six-week plan next time as they pedal back to their world.
They are also a professional rapper and performs a song with Head Pixie during "School's Out!: The Musical". They also had a role in Fairy Idol to try and take the vacant spots of Cosmo and Wanda, who had quit a few days earlier, although after their short rap H.P. commented: "his flow was a little off". They did not win and said something in a rap style that nobody understands before being yanked away by a hook.
Mr. Sanderson competes with H.P. and the rest of the Pixies in the Fairy World Games.