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Cecil Underdunk Terwilliger is the younger brother of Robert Underdunk Terwilliger.

When he was young, Cecil was ignored by his parents compared to Bob, similar to the rest of his life. Cecil's dream since a child was to become Krusty the Clown's sidekick on The Krusty the Clown Show. Cecil spent, according to Bob, "four years at clown college", to which Cecil replies, "I'd thank you not to refer to Princeton that way". Krusty and the producers of his show rejected Cecil during his audition. Cecil offered to perform his prepared "pie-in-the-face take" before he was ushered off-stage, but this did not change the producers' minds as Cecil lacked the dignity necessary to pull the bit off.

Cecil was later released from prison and plotted to flood Springfield by breaking the dam and attempts to kill Bart in the process. Cecil and Bob went to prison even though Bob had nothing to do with it. He later attempted to kill Bart again in a massive plot by Sideshow Bob's family.