William is an English water dragon who thought he was a flying dragon until he found his long lost relative and when he tried fish, he said it tasted better than what he eats, which is apparently humans.
William is a large, green dragon with darker green rings on his neck, yellow eyes and crocodilian snout. Although his nature as a water dragon was only confirmed near the end of the episode, he shows aquatic features from the beginning, such as dorsal fins and webbed feet.
Due to being raised by flying dragons, William was a polite but ruthless consumer of human flesh, and while capable of mercy and making deals with his victims, William will not hesitate to break promises if he feels that the one who he makes the said promises is not fulfilling their part on agreement. Despite this, William isn't sadistic though, eating humans only because he needs to sustain himself and was accustomed to doing so. He also has great lack of self-esteem due to his inability to fly, oblivious to the fact that he is a water dragon. Once he learns his true nature, however, he soon ceases to feel embarrassed and to feed on humans (even freeing Eustace from his stomach at that point).
Being a water dragon, William can't fly, but is quite an adept swimmer, enjoying it right after he tries. While slightly clumsy, William is very strong — capable of crushing a car under his foot, as well as knocking down the windmill with a single bite.
When he was young, his parents lost him in a storm. At some point, he was found and adopted by flying dragons. Despite never enjoying the taste of human flesh, William eventually became accustomed to it. However, he was not able to hide his shame of his inability to fly, believing himself to be a flying dragon.
Some years after he leaving his adoptive parents to live on his own, he finds the Bagge Farmhouse with its residents making a seafood meal, and almost immediately devours Eustace. However when he tries to do the same with Muriel, Courage grabs her and climbs the Windmill, where William is forced to revel that he cannot fly, but Muriel has no time to feel sorrow for the dragon, as he quickly knocks down the Windmill. Courage, however, comes to make an agreement with the Dragon; that if he stops trying to eat Muriel, Courage would help William figure out why he can't fly. But Despite his best efforts Courage was unable to figure out a way to make the dragon fly. Instead, his attempts only cause injuries and humiliation to both of them.
William resumes his chase to devour Muriel, but Courage saves her again by jumping in a lake, where they meet a kind water dragon who greatly resembles William. Realizing that William is also a water dragon, Courage offers him a fish and persuades him to enter the lake. When he does so, William realizes that he actually likes water, and the Kind dragon turns out to be William's brother, who quickly recognizes him and explaines his situation.
Finally realizing why he was never able to fly or enjoy human flesh, William frees Eustace from his stomach and joins their seafood meal. Unfortunately for Eustace, a true flying dragon appears and promptly devours him, to which Eustace exclaims "Stupid Dragon!", ending the episode.