William's brother is a water dragon and his long lost brother. When Courage and Muriel escaped from William, another water dragon appeared and the two screamed. The water dragon said he wasn't going to hurt them. He then offered them some fish fries and fishcicle and he said that it is so good and he liked being a water dragon and not people-eating flying dragons. Courage had an idea and the water dragon hands Courage a fishcicle and Courage offers it to William, but he refused. Courage told him to taste, because William will like it since he hasn't taste it.
The plan worked and William realized his "wings" are fins and that he's a water dragon. William then meets his brother, who explains that William got separated from their parents in a really bad storm, and figures that William must have been adopted by a family of flying dragons when he was too young to realize that he was a water dragon instead of a flying dragon. Learning this and realizing his true identity, William released Eustace from his stomach and ended up sharing some fish and seafood with his brother, Muriel and Courage.