Bun Bun is a lost friend of Pibby. He got consumed by the corruption, or, in correct terms, The Darkness. The Darkness is a glitchy black goo that goes around consuming everyone and everything in its way, turning people from other cartoons into mindless glitches who seek to help the corruption take over. After Bun Bun got consumed, he became a mindless drone to The Darkness and uncontrollably attacks any resistors to corrupt them. He likes it like this... but that's not him saying that...