Mii

A Mii is a customizable avatar used on several Nintendo consoles and mobile apps. Miis were first introduced on the Wii in 2006, and returned on the Nintendo 3DS, Wii U, the Nintendo Switch, and various Nintendo smartphone apps. Miis are based on humans, and they can be created using different body, facial and clothing features. Miis can be used as characters within games on the consoles, either as an avatar of a specific player, such as in Wii Sports, or in some games portrayed as characters with their own personalities. Miis can be shared and transferred between consoles, either manually or automatically with other users over the internet and local wireless communications.

Miis make their Wario franchise debut in WarioWare: Smooth Moves, let alone Mario games in general.

WarioWare: Smooth Moves
In WarioWare: Smooth Moves, the Miis are used as personal files for the players, and are used in several microgames and in the multiplayer mode to represent those playing. The game developers also appear in the credits as Miis wearing different costumes, such as Mario's and Wario's overalls.

WarioWare: D.I.Y.
In the 9-Volt microgame Tightrope in WarioWare: D.I.Y., the purpose is to prevent a male Mii from falling while holding a pole and crossing a tightrope. This microgame is based on the Tightrope Walk minigame in Wii Fit.

WarioWare Gold
Like in WarioWare: Smooth Moves, the player uses Miis for personal files, and the selected Mii is the protagonist in the story. Additionally, the player's Mii appears in the "Tomodachi Life" microgame in 9-Volt's stage.

Other appearances
Though Wario does not appear in New Super Mario Bros. Wii, if player 3 is playing as a Mii, it wears an outfit resembling Wario's shirt and overalls. This was also the case in the unreleased tech demo, New Super Mario Bros. Mii.