Cecil Terwilliger

Television

 
Little Cecil Terwilliger had a big dream:  one day becoming Krusty's greatest sidekick.  As a child he often dressed as a clown & wore makeup in order to practise his future profession.
However, he never managed to step outside the shadow of his big brother Robert, the would-be murderer Sideshow Bob.  When finally he went to a screen test for Krusty the Clown, he was so excited he couldn't speak.  Krusty yelled to get out, but Cecil insisted: he prepared a 'pie-in-the-face' number and wanted to prove Krusty what a great sidekick he would have been.  Sadly, when he was hit with the pie he just showed a nervous smile.   

Krusty shouted that he got it all wrong: this joke is funny only when the guy who gets 'pied' looks elegant.  The clown spotted Bob (who has accompanied his brother for moral support) standing near the stage in a fine suit.  Krusty threw a pie at him, hitting Bob's hat and causing his palm-like hair to spring out.  The clown, laughing hysterically, gave the role to Bob and not to the younger brother, who got only tears. 
 
This is why Cecil hates Bob, and tried to frame him for his own crimes. 
 
Cecil also appears in the episode 'Funeral For A Fiend'.
   

Comics 

In The Simpsons Comics, issue #71, Sins Of The Brothers!, Cecil finally plays the main character part against Lisa Simpson.   When he got out of jail he began thinking about a way to be a shining star in the world of crime, but he found that trying to kill Bart Simpson would have been too clichè. So he decided to murder his sister instead. 
 
However, Lisa had become unsatisfied with her 'little sister' life, and when the two kids were kidnapped by Cecil, she used this topic to get the criminal's attention until police arrived. 
Something strange happened: Cecil found in Lisa a younger mirror of himself.  They shared a hug...  before Chief Wiggum took Cecil away.