Slip is the leader of the Nasties, and the main antagonist of The Neverending Story III: Escape from Fantasia.
He is portrayed by Jack Black (as his first villain role) in one of his earliest roles. He will soon play later on as the voices of Slappy the Dummy and Brent Green (The Invisible Boy) in Goosebumps.
Slip and his gang have been harassing Bastian Bux when the latter is starting high school. Whenever he finds out that whatever happens immediately goes into the book, Slip plans on causing disasters for Bastian in the book. He and his gang then make messes in the library for Mr. Koreander to clean up by himself.
Finally, near the end, using the Auron, Bastian wishes for fighting moves and fights Slip with them. At the end, the next day, all Nasties, including Slip, have turned into friendly students due to Bastian using the story.