Guest A comes as one of the six default Guest Miis that are included in the Nintendo Face Library. Because of this, he technically appears in every single game with Mii support... that's way too many to count! In nearly every single game, he's called Guest A, except for six: Big Brain Academy: Wii Degree, in which he's called "Student A", Mario Kart 8 and Mario Kart 8 Deluxe, in which he's called "Player", Tetris Party and Tetris Party Deluxe, in which he's called "Tetris1", and TV Show King 2, in which he's called "Buggy". Since he's included in the Face Library and not as a file, he doesn't have a filename and consequentially an internal ID.
He has Mingling turned off. His Mii name in his Mii data file is no name, which is the default blank name for Miis.
In his Wii-format Mii data file (extracted from memory), his Mii ID is 80-00-00-00 and his console ID is ec-ff-82-d2 (the same as all default Guest Miis). In his Wii-format Mii data file (July 2021 leak), his Mii ID is d4-0f-87-02 and his console ID is 4f-2b-7f-6c (different from all other default Guest Miis). In his Wii U/3DS-format Mii data file, his Mii ID is 80-00-00-00, and his MAC is EC-FF-82-D2-00-00, very similar to his console ID in his Wii-format Mii data file (extracted from memory).
In his Wii U/3DS-format Mii data file, he was originally created on a 3DS (3).
His Mii data file does not contain any unused data.
As mentioned before, Guest A (Buggy) technically appears in every single game that contains the Nintendo Face Library (AKA, any game that supports Miis), whether you see them or not. Sometimes, they can be tricky to spot, only appearing when the player doesn't have enough or any Miis on their system at all, like in the title sequence for Pilotwings Resort.
Also mentioned earlier was the fact that all but six games call him Guest A. The six exceptions are Big Brain Academy: Wii Degree, in which it calls him Student A, Mario Kart 8 and Mario Kart 8 Deluxe, in which he's called Player, Tetris Party and Tetris Party Deluxe, in which he's called "Tetris1", and TV Show King 2, in which it calls him Buggy. TV Show King 2 is the only game to have given the Guest Miis proper names. The other Guest Mii names are Bee, Hubble, Nugget, Mango, and Shigi respectively.
According to the July 2021 leak, Guest A's race is black.
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Mii Studio code
This is Guest A's Mii Studio code:
You can download Guest A's charinfo-format Mii data file extracted from a save file here.
You can download Guest A's SwitchSDK-format Mii data file extracted from the code of a Switch game here.