From the Super Mario Wiki
Jesters are clowns that work for Booster in his amusement tower. They wear white masks, red hats, and large white gloves. Not only are Jesters a source of entertainment, they can also be used as palace guards to keep out unwanted guests. Although fairly strong and skilled, Jesters will only back up other minions of Booster's in a battle; these clowns never fight with their own kind. In combat, a Jester's primary attack is hitting an intruder with its massive hand. The clown creature can also use special cards (normally needed for magic tricks) to deliver damage. Lastly, a Jester can summon a great fireball, the Flame Stone, to fall from the sky and crash into an opponent, causing massive damage.
Among the unused sprites of Super Mario RPG: Legend of the Seven Stars, there is an enemy named Harlequin, which is a Jester lookalike. The only noticeable differences is Harlequin's green color and the lack of Magic Attacks. Its psychopath thought is "Kekekekekekekekekeka!"
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