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.
Jacks live well-hidden in cannons and spit stars at Kirby if he is 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 can 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 does not attack quickly, it will duck back into the safety of its home.
It is possible Jack's name may come from "Jack-in-the-box", a reference to how Jack comes in and out of his cannon like a Jack-in-the-box.