Mii Costume Guy is the codename for an insider that had been in contact with Papageno's network since about last spring. They got the name because their first (public) act was correctly naming Banjo's Mii costumes two months in advance. Later, he would correctly leak Terry's spirit board, again, two months in advance. Finally, he correctly leaked Byleth's ALTs and stage... In October. Very impressive resume.
Now, in November and December, me and a number of others collected names that they had heard from him or his in-betweens and put them together. At first, we thought it was a "rejected character list" until one of the characters was confirmed in January, Cuphead. These are the Mii costumes that Mii Costume Guy supports that haven't come true yet:
Nintendo: Gooigi, one asked to be kept secret
Capcom: Elecman
Namco- KOS-MOS, Nightmare, Ms. Pac-Man
SEGA: Arle
Square: Mallow, Smithy, Chocobo, one asked to be kept secret
Microsoft: asked to be kept secret
Sony related: asked to be kept secret
Indies: Kris (Deltarune),Knight (Hollow Knight)