Cecil Terwilliger

Cecil Terwilliger is Sideshow Bob's brother.

His first appearance is in the episode "Brother from Another Series". As an homage to the television series Fraiser, Cecil is based on David Hyde Pierce's character Niles (brother of Kelsey Grammer's Fraiser Crane). Cecil felt deeply jealous of his brother, because he wasn't choosen for Krusty's show during the sideshow auditions; the focal reason why they didn't talk to each other for years.

As his introduction story progresses, we discover that he has also hidden criminal behaviours, which have somehow rooted from his jealousy against his brother after what happened in the auditions. As the episode progresses on, Bart and Lisa learn of an embezzlement scheme that will endanger the local hydroelectirc dam that is funded for it, and initally believed that Bob was the one who embezzled it, but Cecil reveals himself to be the true culprit as he is holding Bob, Bart, and Lisa hostage with a gun. It was then he revealed his true colors to frame Bob for the embezzlement scheme and blow the dam with him, Bart, and Lisa inside to get revenge for the audition incident walk away clean with the money.

Fortunately, Bob, Bart, and Lisa went on to work together to stop Cecil in his tracks and turned him over to the police, but unfortunately, Chief Wiggum arrests Bob as well, believing him to be involved. As both brothers are taken away, Cecil tricks Bob into swearing revenge on Bart for not defending his innocence, and as Bob fell for it, he realized he just made Bart his enemy again, and angrily holds Cecil responsible for it.

Cecil has a sister-in-law named Francesca Terwilliger and has a nephew named Gino Terwilliger. Currently, Cecil is remaining in prison, along with Bob, Francesca, Gino, as well as their parents.

Cecil could easily be the most evil villain of the series, tried to flood the city, tried to kill two children (Bart and Lisa) and his own brother. Then, in later chapters, helped his brother to try to kill Bart. And unlike other villains of the series, he contrasts redeeming features. (Making him the only Complete Monster in the entire series