Junior's first appearance is in the episode "The New Ron".
He was voiced by Nestor Carbonell.
Junior and his dad live on a fairly intimidating private island, somewhere off the coast of Europe. Kim first encountered them when Junior's giant tanning lamp was causing rolling blackouts across Western Europe. Upon going to their island and realizing that Senior didn't mean any harm, she gave him a lecture about energy conservation and let him be.
Unfortunately, during that same encounter, Ron commented that several features of Senior's island bore strong resemblance to a villainous lair, and gave him more suggestions for making his home more villain-worthy (Spinning tops of doom, self-activating lasers, speedboat for quick escapes, etc.). Senior originally dismisses Ron's suggestions, saying that they sound too sinister, but upon reflection, he decides that he needs a new hobby, and pretty much drags poor Junior along for the ride for the rest of the series.
In the episode Homecoming Upset, Junior attempt to find the girl of his dreams, "the right combo of shallow, selfish, and insensitive, but still hot enough" in a girl. He hired a person named Ricky Rotiffle to use his special grid, but later finds one in Bonnie Rockwaller, who start to develop some feelings for him. In the series finale "Graduation", Bonnie is upset to learn that she need to undergo summer school after blowing her chance of receiving extra credit needed to graduate. However, Junior, who attended the ceremony, cheers her up by that saying that his heart will still be her as she finishes up summer school, much to her delight.
Junior is, for the most part, interested only in himself. If he put his mind to it, he would likely be a better villain than his father, as he favors simple solutions over more complex "traditional" schemes, but he is also extremely lazy. His dream is to become a pop star, although it is shown in several episodes that he has no musical talent.
Junior wears a tight yellow t-shirt with a red belt and tight black pants.