Miis are the avatar of your character on the Wii, Wii U, 3DS, and Switch Nintendo systems. On Miiverse, every user you see has a Mii, unless their Mii was blocked by the Admins or they somehow don't have one.
To make a Mii, just go to the Mii Maker application, and get your creative mind ready. Miis have a lot of different choices, such as hair, skin, eyes, mouth, nose, eyebrows, colour, height and weight. However, some users believe that these choices are still quite limited. Users can also use the Wii U or 3DS camera in Mii Maker to take a photo of themself and have it turned into a Mii.
Outside of Miiverse
Super Smash Bros. for Wii U and 3DS
They are also playable characters in Super Smash Bros. for Wii U and 3DS. You can choose from a Brawler, Swordsman, or Gunner, select their equipment and special moveset, and put them in various costumes and hats.
In the aforementioned Tomodachi Life, the player can make Miis of anyone they can think of and customize them in various clothes, hats, additional hair colors, along with giving them a voice, personality, shipping them with other Miis, getting them married, and having Mii babies, which takes parts of the parent Miis' faces. This can result in some bizarre default babies. This can be edited when the baby is first born, but never again.
Many people make Miis based on celebrities and fictional characters. Many people also make original Miis or at least Miis that look like them.
Tomodachi Collection is the first Tomodachi life game for the DS, which was only released in Japan. It's basically the same as Tomodachi Life, which you can also transfer miis from your Wii
MiitopiaMiitopia is a RPG similar to Tomodachi Life with your Mii's growing in friendship, you can use Miis to fill in the rolls of major characters like a king, princess, a genie, and more. The townspeople would be randomly assigned.
The Guests are a list of generic-looking default Miis that can be used if you don't have any Miis and show up at the very end of Mii Selections in games. In Mario Kart 8, all 6 Guest Miis are simply called "Player." Some Miiverse users tend to make their Miis like a Guest, which is generally considered pointless by others. In Super Smash Bros. for Wii U/3DS, The default Mii Fighters also resemble three of the Guest Miis in a way, particularly the similarities between the Guest B Mii and the default Mii Swordfighter.
Guest Miis Gallery