A Mii is a customizable avatar created by Nintendo. They can be used on the Nintendo Wii and any main Nintendo console released afterwards, as well as some apps released by the company.
Mii characters are the characters and the namesake of Miitopia. When the player starts the game, they are prompted to make a Mii, which is cast as the Hero. The player can also choose a Mii from their Tomodachi Life save data if there is any. After the beginning cutscene of the game, the player goes to Greenhorne and can cast the residents as using their own Mii characters, with the Tomodachi Life save data, a QR code, or from the Mii Central if SpotPass is activated. In the Nintendo Switch version, casting characters are always done manually by the player using Mii characters either stored in the Nintendo Switch system, stored in their Miitopia save data (up to 500 Mii characters are kept here), or received from either registered Friends, via access keys, or downloaded from Popular, the latter of which uses the Nintendo 3DS version's Mii database containing Mii characters considered "popular" according to the Questionnaire. The only exception is the Travelers' Hub visitors, which automatically draw from the player's save data or, if enabled and the player has access to the internet, Popular Mii characters.
The player's first Mii is the only Mii they can control during battles since he/she will be the main protagonist. The other members of the party act on their own in battles, though they can be controlled in the overworld if the Hero is not present.
Certain Mii characters, such as the Fab Fairies, the Great Sage, the Ex-Dark Lord, and anyone from the Traveler's Hub may come and stay in the party temporarily. They will use attacks similar to the attacks of their supposed job class (i.e. the Youngest Fab Fairy will use the Dancing Arrow attack, which is actually an attack used by the Elf). They will not take up room at the Inn, though they will appear in the corner of the screen that signifies they are staying at the Inn (Nintendo 3DS) or in a separate room to the right of the Inn (Nintendo Switch).
Miitopia also has its own default set of Mii characters outside the ones used as Mii Cast defaults. These are called "world Mii characters" internally and they are taken from Wii Sports Club or Wii Party U CPU characters. The following lists the world Mii characters.
- In the Nintendo 3DS version, if for some reason Mii data is missing (this is more noticeable for players new to Nintendo 3DS emulation), all Mii characters' heads will be rendered as squares with a red "no entry" sign. This also applies to all other Nintendo 3DS games that use Mii characters.
- If the player deletes a Mii who was featured in a cutscene, the cutscene will replace the deleted Mii with Guest A and he will be named "NoName".
- However, In the Nintendo Switch version, if the player deletes a Mii who was not cast as a party member (e.g. Cheery Granny, Worried Explorer, Scholarly Pioneer), the cutscene will use the default Mii instead.
- According to Nintendo, the proper plural for "Mii" is "Mii characters", though the word "Miis" is used in the European Nintendo website and Play Nintendo.