Justin Cavallero, who was once a part of the Kids Next Door as Numbuh 50, is one of Chad Dickson's friends who is also a member of The Teen Ninjas. He appears in "Operation: E.L.E.C.T.I.O.N.S." with a speaking role: he tells Chad that the Gallagher Elementary students have no chance against their Hendry Middle School forces, and then they are both sent back to their seats (being in a classroom).
Later during the battle between both schools, he interrupts Chad as he's sweet-talking a girl with news that they might have a problem defeating the elementary kids. Then he is told to ready Chad's bike - and not to "scratch it or nothing".
He also appears in "Operation: T.R.E.A.T.Y." when Chad and Numbuh 1 are in a Teen hangout underneath a mini-mart. He reminds Chad that no kids are allowed to know about their hangouts, so he and several other Teen Ninjas chase him out. They intend to shoot him out of the sky; when Kids Next Door aviators tell him to stand down because they're in a "no-fly zone", he tells them that the guys they're chasing (Chad and Numbuh 1) stole his little brother's bike. It is unclear whether or not this was a lie, or if he really does have a little brother.
After this, he makes no further appearances, and his voice is provided by Rob Paulsen.
Recurring Villains: Toiletnator | Mr. Wink & Mr. Fibb | Count Spankulot | Gramma Stuffum | Stickybeard | Knightbrace | Common Cold | Mr. Fizz | Crazy Old Cat Lady | Professor XXXL | Chester | Henrietta Von Marzipan | Mega Mom & Destructo Dad
Adult Villains: The Proper Patrol | Mrs. Goodwall | Mr. Mogul | The Great Puttinski | Nurse Claiborne | Cheese Shogun Roquefort | Coach Wetterhahn | Mrs. Dirt | Robin Food | Dodgeball Wizard | Black John Licorice | Principal Sauerbraten | Principal Smelling
Child Villains: Madame Margaret | King Sandy | Six-Gum Gang | Interesting Twins From Beneath the Mountain | James Nixon McGarfield | Anna Worthington | Valerie | Annoyingly Cute Triplets Who Lived Upon the Hill | Numbuh 30c | Munshi Saban | Windsor | Girl Squad | Crayon Boy