Sylvester's trademark was his sloppy, stridulating lisp (which, like Daffy Duck's, was based on producer Leon Schlesinger's). His sloppy voice was provided by voice acting legend Mel Blanc. Blanc reveals in his autobiography that Sylvester's voice and Daffy's were identical, but Daffy's was speeded up in post-production.
Sylvester is an adorable cat who shows much pride in himself, and he never gives up. Despite (or perhaps because of) his pride and persistence, Sylvester was definitely on the "loser" side of the Looney Tunes winner/loser hierarchy. His character was basically that of Wile E. Coyote while he was chasing mice or birds. He shows a different character when paired with Porky Pig in explorations of spooky places (in these cartoons, he basically plays the terrified Costello to Porky's oblivious Abbott.) Sylvester's most developed role is as hapless mouse-catching instructor to his dubious son, Sylvester Junior, in which the "mouse" is a powerful baby kangaroo. His alternately confident and bewildered episodes bring his son to shame, while Sylvester himself is reduced to nervous breakdowns.