He is the leader of the candy monsters, Irwin Irwin's arch-nemesis, and Hoss Delgado's archenemy. He at first appeared as Mindy's assistant, but later became the film's main villain when it was first revealed that he was Hoss Delgado's evil stepfather who haunted him as a child, and made him hate all monsters. Later, Bun Bun put the witch worm in Mindy's hair that turned her into a witch in the first place, gave Billy his fear of spiders, and made him hate his "son" Jeff the spider, and sawed off Fred Fredburger's tusks while he was asleep (although Fred either didn't notice, or didn't care). All of that was revealed within a span of a few minutes and was given no reason whatsoever, being a parody of the common plot device of a villain being involved in the hero's backstory and harming them in the past, and was taken to implausible lengths.
At the end of the film, General Reginald Skarr seemingly joined Bun Bun's side, but then betrayed and kicked him into a giant pot of hot chocolate where he melted. Bun Bun then appeared in a cloud of vapor and angrily yells: "What did I ever do to you guys?!" before he vanished into smoke, never to be seen again and ending his threat for good. The next day, the Underfist team is awarded by President Mandy for their role in saving not only Endsville, but the world, from Bun Bun's clutches.
During the film's credits, a similar bunny to Bun Bun, but with a blue bow tie and made of flesh, can be seen carrying explosive Easter eggs in a scene jokingly dubbed as "Underfist Easter Beatdown". Whether this bunny is somehow related to Bun Bun is unclear.
- Despite just making one appearance in the whole Grim & Evil franchise, Bun Bun can be considered the unseen Bigger Bad of The Grim Adventures of Billy & Mandy, and perhaps the main antagonist of the franchise as a whole. Due to his actions, Bun Bun caused Billy to get arachnophobia (thus causing Billy to despise Jeff, his adoptive spider son), scared Hoss Delgado to the point of making him hate monsters and supernatural creatures, sawed off Fred Fredburger's fangs and turned Mindy against Irwin by tricking her into believe that Irwin transformed her into a witch (and possibly condemning her to remain so forever, as Mindy is never shown to either remain a witch or get back her human form at the film's end).
- So far, Bun Bun is notable for being one of the few seriously competent baddies in the series apart from Mandy. However, while Mandy is a main character in her own series, Bun-Bun was just a one-shot character, yet Mandy is a lot more dangerous than Bun Bun.
- There was a rumor that Bun Bun was going to make a comeback in the cancelled Underfist: The Series spin-off show (as Underfist: Halloween Bash was intended to be the pilot for such series before Maxwell Atoms' contract with Cartoon Network expired) reincarnated as a real bunny made of flesh due the appearance of a similar bunny during the film's credits sequence, which hinted some plot points, stories and characters which were to be used in the series.