Professor XXXL (pronounced Triple Extra-Large) is an insane scientist whose main goal is to create the perfect snowcone. He is very physically large and is constantly afflicted by mutations which give him various animal parts. The character is unique in that he has never shown to have any intentions of destroying the Kids Next Door or terrorizing kids; in both episodes which feature him as the main villain, he is thought to be carrying out an evil plan by Sector V and is revealed at the end to not be. However, he is more than willing to destroy the KND when they attempt to interfere. This misunderstanding is likely caused by the openly sinister personality with which the professor carries out his harmless endeavors. The one time his intentions were actually evil was in Operation: S.N.O.W.G.L.O.B.E. (Cartoon Network Block Party #8), when he attempted (successfully) to freeze the Earth itself into a snow cone (though he forgot to add syrup). His weapons are usually based in his animal mutations such as if he had octopus legs he would make an octopus based weapon.
Professor XXXL was originally created for Tom Warburton's unproduced series Kenny and the Chimp, and appears in Diseasy Does It or Chimp N'Pox the only episode ever made. This cartoon was included the first episode of Codename: Kids Next Door. In it, he runs a laboratory containing hundreds of deadly diseases such as the "Swine Flu" (only his version causes the victim to grow a pig's head) and SalmonEllaFitzgerald (this version causes the victim to suddenly and uncontrollably break out in song). He tasks Kenny and his pet Chimpy, whom he believes to be Kenny's brother and believes to be the smart one, to look after his lab while he attempts to destroy the PTA, who he is mortal enemies with for some absurd reason. However, he fails and his lobster arm is torn off by the PTA and served for dinner with butter.
In his first KND appearance, Operation: A.R.C.T.I.C., Professor XXXL was supposedly making a machine to eliminate snow days; after Numbuh 30c betrayed the KND, it was revealed that XXXL was in fact working to invent the perfect snowcone, and that the weather machine was being made by another scientist, Bob.
In Operation: Q.U.I.E.T., he bursts into the KND's treehouse loudly to announce that he has successfully created the perfect snowcone (amidst several other uninvited guests who attack the treehouse while Numbuh 1 is asleep). Unfortunately for him, Numbuh 5 grabs it from him and throws it at the Toilenator to prevent him from waking up Numbuh 1.
In Operation: C.O.L.L.E.G.E., Numbuh 1 believes that adults are prematurely accepting children into college so that they can remove their brains, and goes to a local university to address this supposed emergency while the rest of Sector V takes a day off at the beach. There, he is kidnapped by Professor XXXL, who claims that he intended to lure Numbuh 1 to his lair all along, but rather than intending to remove his brain, actually needed him to test his latest snowcone, as he suffers terribly from brain freezes, and needed Nigel (who is immune to them), to try the various flavors out. However, before Nigel can tell him the results of the taste test, his teammates arrive to rescue him, ignoring his attempts to explain the true nature of the situation. The Professor attempts to stop them from stealing his test subjects, but is defeated. At the end of the episode, Numbuh 1 is seen lamenting the idiocy of his teammates, revealing that the snowcone was perfect.
Despite his non-malicious nature, Professor XXXL is frequently seen at villain gatherings, including those in Operation: A.F.L.O.A.T., Operation: U.T.O.P.I.A., Operation: M.U.N.C.H.I.E.S., Operation: A.W.A.R.D.S. and Operation: Z.E.R.O.. This may mean that he hates the Kids Next Door due to their repeated interferences in his experiments.
According to the episode Kenny and the Chimp: Diseasy Does It or Chimp N'Pox, he likes ice cream cake. Also he is noted that the only child he seems to hate is Kenny, the owner of the obnoxious chimp, Chimpy. He has identified several dangerous diseases:
- Swine Flu: A disease that makes your head go inside your neck and a piggy-head replaces it. To cure it, scare the piggy away with a pork product.
- Salmonella-Fitz-Gerald: A disease that makes your head resemble Ella Fitz Gerald (the 1980s Jazz Singer), and sing uncontrollably like her. Surprisingly, the Help-Me-Horrible Disease Help Me Hot Line knows the cure: to lick a chimp, but they don't know where to find a chimp, especially a late afternoon on a Saturday.
In each of his appearances, Professor XXXL has a different animal-based mutation. These include:
- Kenny and the Chimp: Diseasy Does It: Lobster claw for left arm
- Operation: A.R.C.T.I.C.: Turtle Shell
- Operation: C.O.L.L.E.G.E.: Tentacles for legs
- Operation: A.F.L.O.A.T.: Turtle Shell
- Operation: M.U.N.C.H.I.E.S.: Snail shell and tail
- Operation: A.W.A.R.D.S.: Walrus tusks
- Operation: Z.E.R.O.: Beaver tail and teeth.
- Operation: I.N.T.E.R.V.I.E.W.S.: Moose antlers