Jack is a rare enemy exclusive to the Mirror World that appears in Kirby & the Amazing Mirror. It lives well-hidden in cannons and spit stars at its enemies. As long as it stays inside their cannon, Kirby is unable to enter it. Thus Kirby must defeat it if he wishes to enter the cannon it inhabits.Jack can not be eaten.
Jack's in-game sprite
However, Jack is vunerable as it can easily be blasted out of their cannons after the fuse is lit. If Kirby chooses to defeat it before he lights the fuse, he must attack it while it has its head poking out above the cannon. But if he doesn't attack quickly it will duck back into the safety of its home.
Jack looks like a blue blob with a gaping hole-like mouth and two squinty eyes. He also has a large yellow dot connected to a thin stalk on his head.