Who is Bart Simpson's arch enemy?

Though young Bart Simpson deals with bullies and other age-peer antagonists, arguably his greatest and most dangerous rival is Robert "Sideshow Bob" Terwilliger. Sideshow Bob, as he is most commonly known, was once a sidekick to Krusty the Klown, whom Sideshow Bob framed for a crime.

After Bart reveals that Krusty was innocent and that Sideshow Bob was responsible for the framing, Sideshow Bob goes to jail and vows revenge on Bart. At times, Bart appears to be in real (cartoon) danger as a result of Sideshow Bob's hatred. Sideshow Bob is shown to have multiple anti-Bart tattoos, making his hatred permanent.