Shy Guys first appeared as common enemies in the Japan-only game Yume Kōjō: Doki Doki Panic, repurposed as Super Mario Bros. 2 for international markets. They simply walk around, and like most other enemies can be picked up and thrown.
Their design features an iconic mask that has two eyeholes and a mouth. As shown by Shy Guy's celebration cutscene in Mario Power Tennis, Luigi is the only one to have canonically seen a Shy Guy's real face. Shy Guys appear in a variety of colors, with red Shy Guys being the most common.
They often appear in spin-off Mario games with several roles. Within the Mario Kart series, Shy Guys made their playable debut in Mario Kart DS: in the download play multiplayer, players can only play as Shy Guy in their standard kart, marked with the emblem of their mask. Starting with Super Mario World 2: Yoshi's Island they became the basic common enemy in the Yoshi games.
Their charging attack in Smash Run appears to be a combination of attacks they could use in Paper Mario and Paper Mario: Sticker Star, namely charging at Mario in the former and hitting him with flailing arms in the latter.
Propeller Shy Guy, a subspecies of Shy Guys, appear as a trophy simply called "Shy Guys".
There are many types of Shy Guys: they've been known to walk on the ground, fly through the air, traipse about on stilts, wear flowers or camouflage, throw balls, swing balls, swing bats, sail on pirate ships, carry fruit, do the Bamboo Dance, ride submarines, play guitar, play tennis, wear red, come in large or small sizes, steal people's toys, cause mischief...
Shy Guys drive karts as Mute City-style hazards on the Mario Circuit stage, that inflict 15%/10% damage to a character when contact is made. They can be hit with attacks or items; once defeated, they start spinning out of control, becoming harmless.
A character in a white mask with black eyes and mouth that appears in a number of Mario and Yoshi games. Although Shy Guys' actions are varied--from flying to carrying objects--their true faces are still a mystery. If only for an instant, who wouldn't want to see an unmasked Shy Guy?
As a stage hazard
Shy Guys appear as stage hazards on Rainbow Road, where, similar to their role in Mario Circuit, they drive karts which can hurt characters. Unlike in Mario Circuit, they cannot be attacked, and they appear in a variety of colors including black and blue.
In Smash Run
Red, light-blue, yellow, and green Shy Guys appear in Smash Run as enemies. Light-blue Shy Guys have more health, red ones deal more damage, green ones are faster, and yellow ones jump higher. When approached, they 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 its specialty: Defense, Attack, Speed, and Jump respectively.
These bashful baddies are so shy that no one's ever seen their real faces. They debuted in Doki Doki Panic before becoming a Mario series mainstay when that game was reworked into Super Mario Bros. 2. They come in different colors, each with unique characteristics. Some can walk on stilts; others are always on fire!
As a stage hazard
A Shy Guy also appears as a Spirit. In its Spirit battle, the player faces a Mii Brawler, Mii Gunner, and Mii Swordfighter wearing the Shy Guy Mask and outfit. The battle takes place on the Battlefield version of Mushroom Kingdom 2. The enemy favors neutral specials and is distracted by items. The Spirit has a power of 2,600. The Spirit itself is a Novice Neutral Spirit. When equipped, it gives the player better damage with fist attacks and has a cost of 2 slots. It is Spirit No. 43.