Dr. Diaper - An evil scientist who wears a diaper.
The Talking Toilets- Toilets that can only shout "YUM, YUM, EAT 'EM UP!" and eat people, created when George and Harold wrote it in a comic book, and fed it to a machine that makes living 3D copies of 2D drawings created by Melvin Sneedley.
The Turbo-Toilet 2000- The leader of the talking toilets. He appeared on his own in book 11.
Jennifer - An evil alien.
Zorx and Klax - Jennifer's Friends. They tried to help him take over Earth.
Professor Poopypants- A mad scientist from New Swissland that forced everyone to change their name to what the Name Change-o-Chart 2000 says. He later changed his name to Tippy Tinkletrousers.
Wedgie Woman - Ms. Ribble after hypnosis from George and Harold.
Evil George and Evil Harold - George and Harold's evil twins from the alternate dimension. They first met when a horrible accident with the time machine occurred.
Captain Blunderpants - Captain Underpants' evil twin from the alternate dimension.
Nice Mr. Krupp - The Nice Counterpart of Mr. Krupp from the alternate dimension.
Forms
Books
There are 12 books, the first one is known as The Adventures of Captain Underpants, the next one being called Captain Underpants and the Attack of the Talking Toilets.