In Tomodachi Collection, the game starts with Guest C when creating a new Mii to add as an islander, and gets changed into the default male or female Mii when a gender is selected. Normally, this cannot be seen in-game, but looking into the code and RAM confirms the Mii is Guest C.
In Miitomo, he appears as Mr. Tomo, where he has a a darker skin.
In Super Smash Bros. for the Wii U & 3DS, he is used to showcase the design of Mii Brawler. he has black, messier hair and his eyebrows use a different design and are thinner.
In Super Smash Bros. Ultimate, the default Mii Brawler is based on Guest C. However, a few more changes were made to his design. This time around, the Mii Brawler has a skin tone that is slightly paler than Guest C's, but darker than the palest option. The Mii Brawler has long black hair, and while his eyebrows share Guest C's design, but is shifted down by 2 and are thinner. His nose is smaller, and he is sporting a serious frown, which is also smaller and thinner than Guest C's mouth. The promotional Mii Brawler can be unlocked by saving him in World of Light's Dark Realm. Guest C himself, meanwhile, appears as an opponent in certain characters' classic modes, like Bowser's. In there, his voice is randomized.
Guest C's face image.
Guest C's body image.
Mii Brawler (Super Smash Bros.)
Mii Brawler, who is based on Guest C, and Na-rae in the Mii Fighter Creation Menu (Smash 3DS).
Super Smash Bros. Ultimate's Mii Fighters. Like in 3DS, Mii Brawler is based on Guest C.