Mii

About
Miis are characters that you can create and customize via Nintendo's Mii maker which is available on the Wii, Wii U, 3DS, Switch, and A web browser version. Miis are typically used to represent you, family members, friends, etc.

Usage on Miiverse
Miis were used as your profile picture on Miiverse. If you posted something you could select an expression for your mii to have. These expressions were: Default, happy, sad, angry, winking, or shocked.