Mii

Miis are the digital avatars that appear in the Mii Channel. Miis are based off of human beings, and players can design what their Miis look like and use them in compatible games. Shigeru Miyamoto had wanted to create digital avatars for Nintendo systems since the NES and kept trying to create them with every system released, such as the Nintendo 64DD add-on and Nintendo DS. Players can start from scratch or create a Mii from an existing template Mii. Only 100 Miis fit in one Wii, while each individual Wii Remote can hold 10 Miis. Miis can also be created on the Nintendo 3DS. They can be created with 3D photos or AR cards and later edited.

WarioWare: Smooth Moves
Miis appeared with Wario in WarioWare: Smooth Moves. In this game, the Miis are used as personal files for the players, and are used in several microgames and in Multiplayer Mode to represent those playing. One of the multiplayer modes starring the Miis is very similar to the mini-game Balloon Bang. The creators of the game also appeared in the credits as Miis, but were wearing costumes unable to be used by the player, such as Mario overalls.

WarioWare D.I.Y.
In the microgame Tightrope (one of 9-Volt's microgames), the purpose of the game is to prevent a male Mii from falling while holding a pole and crossing a tightrope. The Mii is leaning to the left, as if he's about to fall. The player needs to tap the right side of the Wii Balance Board at the bottom right corner to give the Mii balance. The Mii then leans to right and the player has to tap the left side of the Wii Balance Board. If the player taps the side that the Mii is leaning to (e.g. tapping the left side while the Mii is leaning to the left and vice versa), the Mii falls to his doom. If the player takes too long to choose, the Mii also falls. If the player prevents the Mii from falling, the game is won. However, if the Mii falls, the game ends in a loss.

This microgame is based on the Tightrope Walk minigame in Wii Fit.

Mario Party 8
In Mario Party 8, Miis are playable characters in the Extra Zone and are sometimes seen in the background of mini-games. The Miis in this game, however, wear overalls, gloves, and shoes like the Mario brothers. The shirt they wear depends on their player color, not their favorite color. Miis are also available to personalize players' files.

Mario & Sonic at the Olympic Games
In Mario & Sonic at the Olympic Games, Miis are playable characters, just as capable of competing as the characters from the Mario and Sonic series. Their stats are all five out of ten. In this game, the Miis have no voices and are simply called "Mii" during events.

Mario & Sonic at the Olympic Winter Games
Miis return in Mario & Sonic at the Olympic Winter Games. This time, Miis can be customized to wear different clothes and accessories. Unlike the last game, the stats differ depending on the gear. They can even wear different costumes of the playable characters in the game. Some of the bosses from Festival Mode also give off their costumes for Miis to wear once defeated.

Mario Kart Wii
In Mario Kart Wii, Miis first appear as background characters found in various race courses. They also appear in posters and other signs, as seen in Coconut Mall. Later in the game, Miis can be unlocked as playable characters. There are two outfits available for Miis and both outfits have to be unlocked. The first one (Outfit A), which is available when the Mii character option is unlocked, is a plain racing suit of the Mii's favorite color. The other (Outfit B) depends on the gender of the Mii. If the Mii is male, he receives a Mario outfit of his color, and if the Mii is female, she receives a Princess Peach outfit of her color.

A kart's color scheme changes depending on the Mii's gender (primarily blue for males, primarily pink for females). Miis also receive generic voices in Mario Kart Wii, which has a unique voice for each gender and favorite color combination. Miis of the same gender with the same favorite color will have the same voice recordings, but depending on the weight class, the recordings will be altered to a higher or lower pitch for light and heavy Miis, respectively. The weight class for the Mii driver depends on the size in the Mii Channel. Notably, when the player plays as a Mii, statues and pictures in various courses, such as Moonview Highway, Luigi Circuit, Coconut Mall, and Daisy Circuit, will replace the character heads with that of Mii heads, consisting of the Miis saved on a players Wii console.

A Mii can be either a large, medium, or lightweight character, depending on its height and weight. In Mario Kart Wii, even if a Mii is really tall, but doesn't weigh a lot, it will still be categorized as a large character, unless it's skinny, where it qualifies as medium.

Dr. Mario & Germ Buster
In Dr. Mario & Germ Buster, Miis are able to replace Dr. Mario as the playable character. Here they must maneuver Megavitamins tossed into a glass jar in order to destroy the Viruses. In order for a virus to be destroyed, it must be connected to at least one Megavitamin of the same color. Unlike Doctor Mario, Miis speak. Miis can tell the player how many minutes have passed and how many capsules they've thrown.

Mario Super Sluggers
Miis are playable characters in the game Mario Super Sluggers. Miis cannot be chosen as team captains, but up to 8 Mii's can be used. Mii stats in this game are 6/10 and chemistry with other characters is based solely on favorite color, e.g. red Miis have chemistry with Mario, green Miis have chemistry with Luigi (but oddly, not Yoshi; Yoshi has good chemistry with light green Miis instead), pink with Peach, etc. Miis of the same color but opposite genders also have chemistry with each other.

It should also be noted that all male Miis are right-handed batters in this game, while the female Miis are left-handed batters. However, both genders are right-handed fielders. For some reason, they don't speak in the game.

Mario Sports Mix
Miis are one of the nineteen playable characters in Mario Sports Mix. They're the all-rounder type. Despite their height and width, they all receive the same stats: 3/5 in every stat. The color of their outfits depend on their favorite color and their clothing depends on their gender; male Miis receive a sporting outfit, while female Miis receive a skirt.

Their special ability is very similar to Mario's special ability; they throw the object into a goal which can be blocked by a struggle of the opponent. In dodgeball, the shot causes a damage radius that hits anyone inside the radius. Miis receive voices in this game, a quality somewhat rare in games they appear playable in.

On a side note, Miis never appear as opponents in tournaments.

A real life Mii made an appearance in various commercials to help promote the game in North America. They are sports anchors during these commercials, showing gameplay footage to the players in various segments like the Super Top 3 Show, for example. They also announce the game's rating at the end of these commercials. Also, they announce a press conference on the day of the game's release.

Other Appearances
Miis also appear in games such as Donkey Kong Barrel Blast, and Mario Strikers Charged Football to represent the player, similar to an avatar. In Super Mario Galaxy and its sequel, players can personalize their files with Miis. This has also been done with other non-Mario related games. In Super Smash Bros. Brawl, players can use their Mii on Wi-Fi to represent themselves. Also, Miis can be seen as audiences in Mario Sports Mix and Mario Kart for the Nintendo 3DS.

Check Mii Out Channel/Mii Contest Channel
One of the channels in the Wii is the Check Mii Out Channel (or the Mii contest channel), where Nintendo holds contests to see who can create the Mii that looks the most like a certain person, for example a few contests had players to create Miis that looked like the Tooth Fairy, or another that even had players to create a Mii of a Gingerbread Man. Some of the contests have been Mario related. Here is a list of them:
 * Mario, without his hat, was the first contest in the Check Mii Out Channel.
 * Luigi, without his hat, was a later contest.
 * Another contest had players create Princess Peach.
 * A recent contest was "Someone like Mario, but a little different".
 * One contest focused on Bee Mario.

Trivia

 * Miis receive voices for the first time in Mario Kart Wii, not counting small background noises that they first made in Wii Play ' s Shooting Range event.
 * Miis get their name from the word "me", in the same way that the Wii gets it's name from "we".