Dr. X (as revealed in Black and White, Professor Cranston T. Utonium) is the main antoganist of The Powerpuff Girls Crossover and a main character in Black and White.
Powerpuff Girls Crossover
Dr. X was first seen when he was watching the fight between the Powerpuff Girls and Dexter.
Later he is seen when Bell came home with Mojo Jojo. He explained Mojo Jojo about their plan to destroy the Earth but that only there was one thing in their way the Powerpuff Girls. He said he was gathering people like Mojo to make a giant army of them.
For the last time he is seen when he gave Bell an assigment to go to the moon and activate a giant machine that was part of their plan.