Which Mario Kart game introduced Mii characters as racers?

Mario Kart Wii introduced Mii characters as playable racers.

Released for the Wii in 2008, Mario Kart Wii allowed players to race as Miis saved in the console's Mii Channel. This connected the racing game to one of the Wii's defining features: customizable avatars that represented the people using the system. Mii racers could be unlocked in different outfits and varied in weight depending on the selected Mii.

Mario Kart Wii's roster contains 24 playable characters and includes 32 courses, split between 16 new tracks and 16 returning retro tracks. The game also introduced motorcycles, motion-control steering with the Wii Remote, and several new character additions such as Baby Peach, Rosalina, and Funky Kong.

Mario Kart 8 also supports Mii racers, but it did not introduce them. Mario Kart DS and Mario Kart 7 feature Miis in other ways or support system avatars, yet they do not make Mii characters playable racers in the same direct sense as Mario Kart Wii.