Jack is a rare enemy exclusive to the Mirror World that appears in Kirby & The Amazing Mirror. It lives well-hidden in cannons and spits stars at Kirby if he's near. As long as it stays inside its cannon, Kirby is unable to enter it. As such, Kirby must defeat it if he wishes to enter the cannon it inhabits. Jack may be inhaled, unlike Mirra, the one other enemy that specifically prevents inter-room travel.
However, Jacks are vulnerable, as they can easily be blasted out of their cannons after the fuse is lit; this, however, will not prevent Jack from re-spawning if the cannon goes out of view: unless the fuse is re-lit while the cannon is still in view, Kirby must defeat Jack personally. 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.