Mii

A Mii is a customizable avatar that can represent you in some Wii games and online features.

In the Sonic continuity, some games allow a person's Mii to participate in the game, instead of the default characters. In other Sonic games, Miis can act as an icon for your save file, so you don't get them mixed up with others. In some games they talk such as Mario Kart Wii.

Mario & Sonic at the Olympic Games
Miis can be used in any Olympic event. All Miis wear the same athletic outfit, but in different colors depending on the specific Mii. All the Miis share the same animation and poses with Mario. The announcer refers to every Mii as simply "Mii" rather than their unique names. The Miis themselves are completely silent, having absolutely no lines whatsoever. Their stats are all five out of ten.

Mario & Sonic At The Olympic Winter Games
There is a lot more to do with your Mii in this game than there was in this game's predecessor. Using your earned star tokens, players can purchase many different styles of shoes, gloves, shirts and head gear for your Miis to wear. In addition, you can obtain full costumes of the characters from both Sonic and Mario series to dress your Mii in. Miis all have the same stats, and they have the same moveset as Mario by Default. However, if a Mii's outfit is customized with a character suit or certain gear, their stats will increase. They still have no speaking lines.

Mii Sonic Costumes Mii Mario Costumes
 * Mario
 * Luigi
 * Princess Peach
 * Princess Daisy
 * Yoshi
 * Wario
 * Waluigi
 * Bowser
 * Bowser Jr.
 * Donkey Kong
 * Bee Mario
 * Fire Mario
 * Birdo
 * Dry Bones
 * Dry Bowser
 * Diddy Kong

Sonic & Sega All-Stars Racing
Miis can race in the Wii version Sonic & Sega All-Stars Racing, your Mii drives the Super Zoomer and it's All-Star Move is the Parade Power where four of your Miis pick up your car and start running, knocking out anybody in the way.
 * For the Xbox 360, Miis are replaced with Avatars. The Avatars have the same All-Star Move as the Miis.

Sonic Colors
Miis are be playable in the Co-op feature of Sonic Colors In the Game land zone, they can use all of Sonic's/ Robo Sonic's powers and can use the Wisps powers. Their appearence is the same as the selected Mii, same head and arm shape but withs sonic's upper body and shoes. The voice when selected a color Wisp remains the same. As seen in Sonic Colors Game Land Zone Acts 1-3 (Miis used in Act 2 )by Youtube user Jacktheinfinite101. Same as with Robo-Sonic, should one jump on the Mii's head, he/she will become dazed. A Mii can also be selected as your icon on your saved file.

Super Smash Bros. Brawl
In this Nintendo game that guest stars Sonic the Hedgehog as a playable character, the player can use their Mii on Wi-Fi to represent themselves.

Samba De Amigo Wii
In this maraca shaking musical game, Sonic and some Chao make very energetic dancing cameos in a Green Hill Zone themed stage. Also present in Green Hill Zone are the player's Miis who are more than happy to join in on the party.

Sonic Unleashed
In the Wii version of Sonic Unleashed the player can use a Mii to serve as their icon on their save file.

Trivia

 * A Mii's hands are round spheres, but in the Mario & Sonic series, Miis have opposable thumbs.
 * Miis' physical apperance has been known to vary, in Wii Sports Miis (in most cases) had no legs, but in Wii Sports Resort they have legs. In Wii Sports (in most cases) they had no arms, but in Sonic and SEGA All-Stars Racing, they have arms.
 * Although Miis can be either gender, in Mario & Sonic at the Olympic Winter Games, a male Mii can be given a female's outfit and vice versa.
 * Although you can customize the weight and height of your Mii, all Miis in Sonic games will have the same height and weight regardless.
 * If you select your Mii as a favourite in the Mii Channel not only will they have red pants but in Mario & Sonic at the Olympic Games part of the Miis outfit would be red as well (i.e. wrist bands)
 * It is possible that Miis could have super-human speed and strength, considering they compete on an even level with the other default characters in Mario and Sonic at the Olympic Games and Mario and Sonic at the Olympic Winter Games.
 * In Sonic & Sega All-Stars Racing, the Mii's All Star Move is just like the Avatar's All Star Move.