Shy Guys made their debut in the Japan-only game Yume Kōjō: Doki Doki Panic (repurposed as Super Mario Bros. 2 for international markets) as a common enemy, and have become a common enemy in later Mario games. They are humanoid enemies who wear robes and a white mask that features two large eyeholes and a round mouth hole. Their robes also appear in a variety of colors, with red being the most common.
Shy Guys are also known for being highly secretive about their true appearance, desperately trying to hide themselves in the few times their mask falls off. In Shy Guy's celebration cutscene in Mario Power Tennis, Luigi is the only one to have canonically seen a Shy Guy's real face. The Mario Strikers series also shows that Shy Guys have several masks that display different emotions, which they swap between when they need to.
In Super Mario Bros. 2, Shy Guys are mostly known for walking around and damaging the player on contact. Shy Guys can be defeated by grabbing them and throwing them against another enemy or into a nearby pitfall. Starting with Super Mario World 2: Yoshi's Island they became the basic common enemy in the Yoshi games.
Shy Guys often appear in spin-off Mario games with several roles. They first made their playable debut in Mario Tennis 64. They also appear in Mario Kart DS and Mario Kart 7 in download play multiplayer, where players without the respective games can only play as Shy Guy in their Standard Kart, marked with the emblem of their mask. In Mario Kart DS they're only playable in download play, while starting with Mario Kart 7 they're playable normally.
As a stage element
Propeller Shy Guys, a subspecies of Shy Guys, appear in the Yoshi's Story stage, flying across the screen in groups at random intervals. They cannot harm the players, but can be damaged. Some of them carry food (unless food items are disabled), which they will drop when they get KO'd, which will result in the player receiving a Shy-Guy KO bonus where the player receives 20 points for each Shy Guy KO'd.
As a stage hazard
Shy Guys on their Standard Karts appear as stage hazards on the Mario Circuit stage. They drive their Standard Kart around the track and inflict 15%/10% damage to the player's fighter on contact. They can be hit with attacks or items; once defeated, they start spinning out of control, becoming harmless.
As a stage hazard
In Super Smash Bros. for Nintendo 3DS, Shy Guys on Standard Karts appear as stage hazards on the Rainbow Road stage, reprising their role from the Mario Circuit stage in Brawl; however, Shy Guys cannot be attacked, and they appear in a variety of colors.
As an enemy
Shy Guys appear as enemies in Smash Run, appearing in different colored robes, including red, yellow, green, and light blue. Each different color of Shy Guy features a different function, with red Shy Guys dealing more damage, yellow Shy Guys jumping higher, green Shy Guys being faster, and light blue Shy Guys having more health.
When the player's fighter approaches them, Shy Guys will jump in place a few times before charging at the player while flailing their arms. Each type of Shy Guy drops stat boosts related to their colors and specialty, including Attack, Jump, Speed, and Defense respectively.
The Shy Guy trophy appears in both versions and is part of the Yoshi's Island Trophy Box in the Wii U version. All other trophies are exclusive to the 3DS version.
As a stage hazard and stage element
Shy Guys return as stage hazards for the returning Figure-8 Circuit and Mario Circuit stages, and as a stage element on the returning Yoshi's Story stage, reprising their behaviors from the previous games.
As a Mii Costume
The Shy Guy serves as the basis for the Shy Guy Mask headgear for the Mii Fighters.
In Adventure Mode: World of Light, Shy Guy is presumably one of the countless spirits captured by Galeem during his takeover of the universe. Geno's spirit is located in the hill section of The Light Realm.
Shy Guy's Spirit Battle uses the Mii Brawler, Mii Gunner, and Mii Swordfighter puppet fighters wearing the Shy Guy outfit and is fought on the Mushroom Kingdom II stage's Battlefield form. During the battle, Throwing Type items will appear while the puppet fighters use neutral specials, referencing the projectiles launched by the similar-looking Snifit enemies.
In the Roger the Potted Ghost Spirit Battle, two Shy Guys each use a Meta Knight puppet fighter and is fought on the Dracula's Castle stage's Ω form. They are fought along with the Giant Wii Fit Trainer puppet fighter representing Roger the Potted Ghost. During the battle, the player must defeat the Wii Fit Trainer in one minute and the fighters' jumping power are decreased, referencing the ground-based boss battle against Roger in Yoshi's Island.
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