It will also appear when you finish playing with one of the characters once. Only its silhouette will appear, the character will make a '?' on its head and swim away after it. Characters that make the '?' are: Boris, Layla, Eddie, Peter and Harry,
How to defeat him:Edit
When you see him first, you must bite the red circle, much like to the barracuda. After being bitten, he will try to eat you and like the barracuda, he will let you after some time.
ALWAYS, when you must fight him, there will be mines. Just lead him to a mine. He will be stunned and 5 Shrinkshrooms Bubbles will appear. Each eaten Bubble will decrease Intruder's health. When you fill the bar in yellow, it will get small and the character will swim after him, with the exception of Eddie, which doesn't fight him and Borris, which will eat him.
When playing as Peter, you can just have the chance to eat Shrinkshrooms when a pelican is nearby, making it small and eatable.
When a Shrinkshroom is eaten, it will turn red.
When you first defeat him, he will be truly huge, then smaller and smaller.