Bun Bun is an evil marshmallow bunny and the main antagonist of Underfist: Halloween Bash.

He is the leader of the candy monsters, Irwin's Arch nemesis, and Hoss Delgados stepfather. 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, that 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 motive 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 Skarr 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. He was notable for being one of the few seriously competent baddies in the series apart from Mandy.Judging by what he did to the heroes past, Bun Bun is the true main antagonist of the show.