Mr Popplewick was the identity used by the Valeyard while in the Matrix.
Popplewick Senior, Junior and very Juniour were Victorian Clerks and claimed to be an assistant to "Mr. J.J. Chambers" in the Fantasy Factory. Sporting an explosive quill behind the ear Popplewick was a stickler for procedures which was used to delay the Sixth Doctor from seeing Mr. J.J. Chambers.
Popplewick was held hostage by Sabalom Glitz, and his true identity was revealed by the Sixth Doctor, who pulled off his mask. He declared that he knew as the performance was too perfect. (TV: The Ultimate Foe)