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 like Mario Kart Wii.

Sonic & Sega All-Stars Racing
Mii's can race in 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, Mii's are replaced with Avatars.

Mario &amp; 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.

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

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.