From the Super Mario Wiki
Anti Guys are Black Shy Guys that appear in Paper Mario. They are also known as Deadly Guys by in-game characters due to the fact that their attacks are extremely powerful. They are one of the strongest non-boss enemies that Mario battles, second only to the Amazy Dayzee, and even then, Anti Guys have much higher HP.
In the game, Mario first encounters an Anti Guy in Shy Guy's Toy Box. This Shy Guy is guarding a treasure chest holding a Power Plus Badge. If spoken to, it demands that Mario backs off, but the player can engage it in battle and get the badge by beating it. Later, in Bowser's Castle, the second of three Guard Doors gives Mario a "best three of five" quiz. If Mario meets the given conditions, he can pass. However, if he does not, he has to face three Anti Guys at once, known as the Anti Guys Unit.
Lemon Candy is quite possibly an Anti Guy's favorite treat. Instead of fighting the Anti Guy in Shy Guy's Toy Box, Mario can instead bribe the Shy Guy to leave by giving it a Lemon Candy, allowing Mario access to the Power Plus Badge. However, if Mario does this, he cannot fight the Anti Guy to earn the large number of Star Points awarded upon its defeat in battle.
Black Shy Guys from Yoshi's Story share similarities with Anti Guys, particularly in that both enemies are powerful types of Shy Guys working for the Koopa Troop. The only difference is that Anti Guys are fighters, whereas Black Shy Guys are fliers. Worthy of note is that the masks of Anti Guys are gray, unlike the masks of other Shy Guys, which are white.
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