Burton Plushtoy III is an antagonist, and the only true main villain in the series.
Burton is a obnoxious 10-year-old billionaire who first appeared in "Kenny-Napped". He had set his sights on Kenny, and planned on adding him to his collection of exotic pets. When the Cassidys wouldn't sell, he turned to Plan B: kiddnapping! Luckily, Marty informed Kat of the goings-on, and she snuck into his mansion disguised as a reporter for the school paper doing a scoop on Plushtoy's pets. When asked about Kenny, Burton replied that since the tiger shark wasn't entertaining, he was having him turned into sushi. Kat then revealed her true colors and set Kenny free. Burton's collection of exotic pets was set free, too, and all found good homes afterwards.
Later, in "Ocean's Three", Burton retaliated by buying the Tibberon Aquarium, and began cramming all the fish together to make more room for future exhibits. Upon hearing about this, Kat organized a protest. Kenny attended said protest. Burton quickly noticed and despite warnings from his butler, he intended to once again claim Kenny as his own. Only this time, Kenny was an "inside man" as part of a top-secret plan to free the citizens of the aquarium. While Burton was distracted, Kat and Marty infiltrated the aquarium through the air vents, and intervened just as Burton decided to add a sushi bar to the aquarium attractions, with Elly as its first victim. The health department showed up shortly afterwards, and took Burton away for attempting to open an unlicenced sushi bar.