He was introduced in Operation: MACARRONI, where a prisoner exchange between the KND and Mr. Boss's group occurs involving him and the Toiletnator. As they go back to where Numbuh 13 lives to return him to his sector, he warns them that the adults are planning to have macaroni for dinner, which poses no apparent harm whatsoever and is ignored by the others.
When they learn that they are getting their macaroni from the "Rainbow Monkey Lisa", they head to the Macaroni Museum of art to stop them. Once there, Numbuh 13 completely ruins the mission, destroying a Rainbow Monkey vase as well as the Rainbow Monkey Lisa, and being inadvertently recaptured by the villains by hiding himself in their bag, which the others are actually glad of.
In Operation: I.T., he is buying a hot dog when he realizes that he is "It" and will become Supreme Commander. Even though everyone is trying to avoid becoming Supreme Commander, Numbuh 13 is immediately tackled to the ground by everyone else in the area, who would rather risk being given the job themselves than let Numbuh 13 have it.
In Gamewizard's universe, his name is Billy Horvitz. He first appeared in the one-shot "Useless", where he tried helping Numbuh 202 carry her tray over at the Moonbase Cafeteria, but only tripped. He was depressed, thinking he was too clumsy to do anything right, but Numbuh 11-Teen believed that he had potential, and became his first real friend.
In the future, he and Numbuh 11-Teen have a son named Eric Horvitz.