JoJo is Mandark's inter-dimensional monster which he summoned in order to win the science fair, but later got out of control and started wrecking the place, he later abducted Dee Dee and attempts to eat her. Mandark and Dexter work together to defeat him, Dexter saves Dee Dee with his hand invention and Mandark sends him back to the dimension in which he came from. His vocal effects were provided by Tom Kenny.
- Coincidentally, in The Powerpuff Girls, Professor Utonium has a lab monkey named Jojo who later became the main villain Mojo Jojo.
- The incantation necessary to summon JoJo is "Chim chiminee cheroo!" Which is a song from the Walt Disney film Mary Poppins (1964).