From the Super Mario Wiki
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Knife Guy is a jester and the partner of Grate Guy in the game Super Mario RPG: Legend of the Seven Stars. The two work as entertainers and bodyguards for the demented Booster. When Booster (with Toadstool in tow) flees his tower, Mario and company chase after him. However, the heroes are ambushed on the balcony of the tower by Knife Guy and Grate Guy. Knife Guy is a skilled fighter, but both he and his partner are defeated by Mario.
After their defeat, Knife Guy and Grate Guy decide to split up. Grate Guy opens his own casino near Bean Valley while Knife Guy stays at Booster Tower. Inside the tower, Knife Guy practices his juggling (trading in his knives for colored balls). If Mario returns to Booster Tower, Knife Guy will offer to play a game with him. The jester will juggle one yellow ball with a multitude of red balls; when he stops, Mario has to guess which hand the sole yellow ball is in. If Mario wins he'll be "rewarded" with a Moldy Mush, a Wilt Shroom or a Rotten Mush, and sometimes a Mid Mushroom or Max Mushroom.
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