Cecil Terwilliger

"I forgot to mention, I'm planning to blow up the dam with you inside."

- Cecil to Bob, Bart and Lisa

"At last I'm going to do what Bob never could. Kill Bart Simpson!"

- Cecil about Bart

Cecil Terwilliger is the main antagonist for The Simpsons episodes Brother from Another Series and the secondary antagonist (along with the other Terwilliger family members) for Funeral for a Fiend. He is a younger brother of Sideshow Bob, and son of Robert Terwilliger Sr. and Dame Judith Underdunk. He also has a sister-in-law named Francesca Terwilliger and a nephew named Gino Terwilliger.

He is voiced by David Hyde Pierce.

History
Cecil felt deeply jealous of his older brother Bob, because as children, Cecil was ignored repeatedly by his parents, and instead compared to Bob. As adults, Bob and Cecil didn't talk to each other for years, because Cecil wasn't chosen for Krusty's show during the sideshow auditions. Krusty, Cecil's childhood hero, considered Cecil too unfunny, as soon as he noticed Bob, he chose him as his new sidekick.

As his introduction story progresses, we discover that he has also hidden criminal behaviors, 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 hydroelectric dam that is funded for it, and initially 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.

When Bob was later arrested for attempting to kill the Simpsons again, Cecil helped him to avoid the prison and lure Bart into the trap. After Bob's fake funeral, Cecil came to unhappy Bart who felt guilty for Bob's death, and suggested him to make his peace with Bob before his cremation. This led Bart to visit to the funeral house, only to be locked in the coffin by Bob who then tried to burn him up.

Currently, Cecil is remaining in prison, along with Bob, Francesca, Gino, as well as their parents.

Trivia

 * As a homage to the television series Fraiser, Cecil is based on David Hyde Pierce's character Niles (brother of Kelsey Grammer's Fraiser Crane).
 * 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.