Grimace
The grimace is Rayman's initial line of defence, which he has been capable of right from the beginning of the original Rayman game. To grimace, Rayman stretches his mouth wide with his hands, sticks out his tongue, rolls his eyes and makes a gurgling sound with his throat. The power's functions vary from game to game.
In Rayman

In the original Rayman game, the only use for this power is to scare the tall Livingstones; when they see the grimace, they are driven mad with fear and start to run uncontrollably, usually killing themselves by falling into water. In the first part of the Swamps of Forgetfulness, the grimace is temporarily replaced by the magic seed which Rayman receives from Tarayzan. When he defeats Mr Stone in Mr Stone's Peaks, Betilla grants him the power to run, which permanently replaces the grimace in all versions except the Atari Jaguar and Nintendo DSi versions.
In Rayman 2
The grimace does not appear in most versions of Rayman 2; however, in the Dreamcast, iOS and 3DS versions of the game, Rayman occasionally performs a grimace at the camera if left idle by the player for a time. He also does this in the Game Boy Advance and N-GAGE version of Rayman 3.
In Rayman 3
In Rayman 3, Gumsi uses the Sceptre of the Leptys to grant Rayman this power after he defeats Reflux in the Desert of the Knaaren. Rayman can then use this power to convert Black Lums back into Red Lums by making them laugh after defeating their Hoodlum costumes. It is known as ‘The Power of the Leptys’ in this game. This makes Rayman 3 the only game in the series where the grimace appears but cannot be performed by default, and one of only two games in the series, the other being the original Rayman game, where the grimace has a function in the gameplay. A promotional statue to Rayman 3 which recreates the grimace artwork was done in a limited number to hundreds of units worldwide, very few containing a box and an artwork book with conceptual art of the game.
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Rayman performing a grimace in Rayman 3. In this game, it can be used to revert Black Lums into Red Lums.
In the Game Boy Advance version of Rayman 3
In the Game Boy Advance version of Rayman 3, much like in the Dreamcast, iOS and 3DS versions of Rayman 2, Rayman does not use the grimace to any practical effect, nor can it be willingly triggered by the player; it plays as an idle animation if the player does not give an input for some time. Rayman can also be seen using the grimace to taunt Jano prior to their fight.
In the Game Boy Advance version of Rayman Raving Rabbids
In the Game Boy Advance version of Rayman Raving Rabbids, the grimace behaves identically to how it did in the Game Boy Advance version of Rayman 3, where it will trigger as an idle animation.
In Rayman in the Phantom Show
While Rayman cannot perform a grimace in Rayman in the Phantom Show, the third DLC for Mario + Rabbids Sparks of Hope, concept art for the DLC depicts him grimacing, suggesting that it was considered for addition, perhaps as an animation for winning battles, but was scrapped at some point.
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Concept art for Rayman in the Phantom Show, which depicts Rayman grimacing on the bottom right