Justin used to be a part of the Kids Next Door before he became a teenager. This is evident by a photo of him working to build something alongside other kids in the end credits of the episode Operation: Z.E.R.O..
His first appearance was in Operation: E.L.E.C.T.I.O.N.S., where he's first shown telling Chad that the Gallagher Elementary School-students have no chance against their Hendry Middle Schoolforces--but 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 the news that they might have some trouble defeating the elementary school-students. Chad then asks him to get their bikes (while warning Justin not to get his scratched or damaged). He makes no further appearances in the episode, it was unknown if he joined the battle or not.
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 as he called Numbuh 1 stupid. Numbuh 1 angrily asks who are you calling stupid 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. However what ever the purpose is he needed some help from Kids Next Door aviators to shoot them down
After Operation: T.R.E.A.T.Y., he makes no further appearances, and his voice is unknown.