Knuckle Joe did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
In his debut role, he mostly attacks with various fighting moves like throwing punches and kicking, naturally giving the Fighter copy ability. He is also a helper and appears in Heper to Hero as a main mascot alongside Waddle Dee. He is Kirby's second opponent after Waddle Doo in the Megaton Punch minigame.
Super Smash Bros. series
In the Super Smash Bros. series, Knuckle Joe features prominently as both an Assist Trophy and normal Trophy in every game since Super Smash Bros. Brawl. As an Assist, he uses the Vulcan Jab technique and other attacks to damage the opponents of the player who summoned him.
Trophy appearance from Super Smash Bros. for 3DS/Wii U