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 attempted to eat her. Mandark and Dexter worked together to defeat him. Dexter saved Dee Dee with his hand invention and Mandark sent JoJo him back to the dimension which he came from. His vocal effects were provided by Tom Kenny.
- The incantation necessary to summon JoJo is "Chim chiminee cheroo!", which is a song from the Walt Disney film Mary Poppins (1964).