Sweet William is a large, muscular member of Cat R. Waul's Cactus Cat Gang . He was the minor antagonist in An American Tail: Fievel Goes West before playing a bigger role as the secondary antagonist in Fievel's American Tails. He is voiced by the late Kenneth Mars.
In An American Tail: Fievel Goes West, William appears only as a background character, like most of the other members of the gang. But in Fievel's American Tails he is elevated to a main villain with a gang of his own. It appears that after Waul was ousted from Green River at the end of Fievel Goes West, Sweet William no longer took orders from Waul unless there was something in it for him, and in fact beats Waul up in many episodes. He can be considered even more dangerous than Waul because of his aggressive nature. He plays bigger role in the show. The very first episode, "Fievel, the Lonesome Ranger", has Sweet William forge an alliance with Cat R. Waul after being promised that his scheme will result in them getting to eat mice. However, after the scheme fails, William beats Waul senseless. In one episode, "Mail Order Mayhem", he plays out as the main bad guy, working with two other cats Slim and Felonious without Cat R. Waul's authority (as far as we know). In this episode he orders a "Mouse Slapper" which fires nets on mice, and he terrorizes the town with it, but is easily subdued by hired lawmouse Clint Mousewood. In another episode he reveals a soft side, taking a liking to Yasha Mousekewitz and finding her too adorable to eat when she wanders into his hideout in "Babysitting Blues". In "Little Mouse on the Prairie", he inadvertently rescues the Mousekewitz family from being eaten by Waul, having his gang beat Waul up for cheating him in a gambling game earlier in the episode. In "Bell the Cats " once again he is the main villain, with Waul being absent for unknown reasons, keeping the mice of Green River in the grip of fear until Fievel ties a bell on his tail and the tails of his gang.