Knuckle Joe is a Kirby series character who first appeared in Kirby Super Star for the SNES. He's not widely known as some Kirby characters (Kirby, Meta Knight, etc...) are, though he has managed to gain a fanbase and has even appeared as an Assist Trophy in the Super Smash Bros. series, where once summoned he will pack a massive punch toward your enemies.
Knuckle Joes are actually a species, and in Kirby Super Star a not-so-common one. They're not entirely hard to defeat, though they seem to be masters at martial arts. In Brawl, it shows that they're, or at least one of them, is also able to perform lightning blasts, which like his punches he uses against his, or rather your opponents.
In Kirby Super Star Fighter Kirby can sacrifice his Copy Ability to summon a playable Knuckle Joe as an ally. They were essentially the same as Fighter Kirby, though with a new design, and minus the Tomoe Throw.