Mojo Jojo is the main antagonist of The Powerpuff Girls. He's an intellectually advanced chimpanzee and the arch-nemesis of the Powerpuff Girls, constantly scheming to take over Townsville, and eventually, the world. He is Professor Utonium's former pet.
Mojo was originally Professor Utonium's troublemaking lab assistant and adopted son named Jojo. Due to being ignored by the professor his whole life, Jojo started destroying everything in the lab to notice him as more than just a thing. He pushed the professor into the Chemical X which led to creating the Powerpuff Girls. Due to the explosion, Jojo's intelligence increased so much that his brain began to stick out of his head. It also gave him super strength, and gave him more fur on his body.
Seeing how Professor Utonium gave the Powerpuff Girls the love that he never got, Jojo became jealous and hateful. He left home and decided to seek revenge on them, as well as he changed his name to Mojo Jojo. Mojo became homeless and had to live in a box. Spending time on the street by himself with no one to care about him almost immediately turned him cold and emotionless providing a motive as to why he would care to destroy Townsville and conquer the world. He doesn't always do this or makes plans related to this as in one episode he fell asleep while doing nothing but reading a newspaper (but was still beaten up due to circumstances beyond his control). Mojo is shown to have a civilian life despite his criminal activities; he is often shown walking around Townsville without any citizens running in fear, and even buys groceries legally. The girls also tend to treat him as a friendly neighbor, often coming to his house to ask for favors without attacking him (though they do sometimes).
Near the end of "The Powerpuff Girls Rule!", he finally took over the world but rather than ruling it with an iron fist like he always said he would, he actually solved all of the world's problems showing that he actually isn't evil.