When Mario, Mallow, Geno, and Bowser were chasing after Booster, Knife Guy and his partner Grate Guy attacked them. Mario defeated them both. Afterward, Knife Guy could be found at a spot in Booster Tower, juggling balls rather than knives, and offering Mario to play a minigame to see if he could guess which hand the yellow ball was in. If Mario gets it right at the highest difficulty, he can get the Bright Card required to enter Grate Guy's Casino.
In battle, he has 700 HP, 70 ATK, 55 DEF, 20 magic attack, and 10 magic defense. He specializes in physical attacks. He has less HP than Grate Guy, and is weak against fire. It has often been suggested to defeat him first.
His Psychopath thought is "Happiness is hip!"