Mojo Jojo, formerly known simply as Jojo, is the main antagonist of The Powerpuff Girls franchise.
Jojo was once a trusted assistant of Professor Utonium, but he always made lots of messes, especially when the Professor was trying to create the perfect little girl. But thanks to Mojo pushing him, an inadvertent addition was made: a whole beaker of Chemical X, making three powerful girls. When the Professor pays attention to the girls, Jojo feels left out.
The morning after the girls accidentally destroy Townsville, Jojo reads about them. That night, when the girls are lost and attacked by The Gangreen Gang, Jojo knocks them down and saves the girls. He uses words to show them how he "feels". He tricks the girls into using their powers to build a "Help-the-Town-and-Make-It-a-Better-Place Machine" (his future lair, actually) and give him some chemical X. He "rewards" them by taking them to the zoo, only to use a camera-disguised weapon to mutate all of the monkeys.
Much later, after the Professor is released from prison (where he'd been put for creating the girls), and the girls try to show him the "good" they had done, it was shown that they had helped Jojo (then renaming himself "Mojo Jojo") get Townsville attacked, not only making the townspeople despise the girls even more than when they had innocently wrecked the town with a game of tag, but hurting the Professor. After the girls blast off into space, Mojo kidnaps the Professor.