Sr. X is a thief who proclaims himself "the king of crime", even though his typical hideout is no better than the Beagle Boys' ones. While he is smart, his henchmen are pretty incompetent, and the only other with brains in the gang is his talking pet crow X-8.
While Sr X's crimes are generally centered on getting profit, he mainly wants to steal important treasures to become famous and be known by everyone else as the king of crime. However, he has only managed to be known as "the king of crazy" by the press.
His robberies have made him both an ally and a rival to the Beagle Boys, depending on whether both Sr X and the Beagles are in a mood to share the loot during any specific robbery.