Big Billy is the biggest and the strongest man of the Gangreen Gang in the episode Slave The Day. He is saved by the Powerpuff Girls from the train Big Billy called Choo Choo, and thus he wants do everything for the Powerpuff Girls.
He is amazingly strong. He can destroy the train with his punch in the episode Power Lunch Big Billy's power he is just being a rock (he ate hard candy).
In the episode School Rocked Out it's revealed that his real name is William W. Williams and he is a cyclops (he has only 1 eye)! In the episode Bang For Your Buck the Gangreen Gang steals $100 to buy Mojo Jojo's Destructo Ray and Big Billy buys a teddy bear for only $5!