Joe Agate.

Joe Agate is the main antagonist of the 2006 special, He's a Bully, Charlie Brown, the last Peanuts special to have Bill Melendez involved. He is the son of the owner of the trading post at the summer camp. He is a bully, like it says in the title, who plays beginners and deceives littler children by telling them he is only teaching them. But after he is done, he has tricked them into giving up all their marbles. Particularly, he tricks Rerun Van Pelt, the brother of Lucy Van Pelt and Linus Van Pelt, whose paternal grandfather, Felix, was once a marble's champion in 1939.

By the end, Charlie Brown (the protagonist, of course), who has learned from a disguised Snoopy, how to play marbles, scolds Joe for playing beginners and tricking the kids. He then challenges Joe to a marble's game for keeps. As Charlie Brown has lost, he is given another chance by Snoopy, who has kept two of the marbles. Joe questions Charlie for it, but the latter tells him the marbles are his because Snoopy is his dogs. As soon as Joe tries to show a trick that never misses, it actually does this time, so Charlie wins the last marble, including Joe's shooter. Joe gives up the marbles he had taken from Rerun, and then goes away.