Sideshow Bob Terwilliger is
/Krusty the Clown's silent (during the Krusty show) sidekick. Along the way he snapped and has tried to kill Krusty, Bart, and indeed everyone in Springfield several times, but has failed through a combination of luck and Bart and Lisa's efforts. Was briefly married to Selma (whom he also tried to kill). His voice is that of
Frasier star
Kelsey Grammer. Grammer's costar [David Hyde Pierce]
? made an appearance in one episode as Sideshow Bob's brother Cecil.
Replaced as Krusty's sidekick by Sideshow Mel.