As a recap, "Mii Costume Guy" is a codename given to an insider who has been in connection with Papageno's network since about last Spring. He's been given this name because his first feat that was made public was correctly leaking Banjo's Mii costumes last July. Since then, he has also successfully leaked Terry's spirit board in September, Byleth's ALTs and stage in October (I didn't get this information until December). There's a language barrier (presumably Japanese) and he typically talks through "in-betweens" because of this. Because of that, sometimes there are translation errors like how Terry's spirit board was interpreted as Mii costumes and Byleth's ALTs were thought to be literal Mii costumes.
Since last summer, he's released a good amount of information but a lot of it hasn't come true yet. A vast majority of it has been Mii costumes. Through various conversations with Mii Costume Guy and his In-betweens, these are the Mii costumes he has put out since December:
-Ms. Pac-Man
-Nightmare (Soul Caliber)
-Gooigi
-Elecman
-KOS-MOS
-Cuphead
-Kris (Deltarune)
-Knight (Hollow Knight)
-Arle (Puyo Pop)
-Mallow (Super Mario RPG)
-Smithy (Super Mario RPG)
And four costumes that were recommended to be kept secret:
-One heavily connected to Nintendo
-One heavily connected to Square
-One heavily connected to Microsoft
-One heavily connected to Sony
For the record, Mii Costume Guy hasn't put out anything new since about mid December.
I never would have thought of the mii costumes the guy mentions. At first they seem like random guesses, but now that I think about it, each one sounds quite reasonable.
-Pac-man celebrates 40 years, Ms. Pac-Man has been quite loved since the term "echo" exists.
-Elec Man already had an assist trophy, working with an existing model would be easier than doing it from scratch, he's a very popular Robot Master (also it seems that the megaman series receives a lot of mii gunners for obvius reasons).
-I live in Mexico, and just as King of Fighters is loved here, the same can be said of Hollow Knight, in the Mexican eshop the game costs like 6 or 7 dollars, it is always in the best-selling section, even higher than titles on offer or AAA games, most of my acquaintances and friends love it or at least recognize it.
-When they released the Cuphead Mii they did a spirit event. I find it strange that they have not done the same with Sans yet, maybe they are waiting for Kris's mii to do it.
I hope Geno is part of Volume 2, and if it isn't, it would be cool if it was that secret square mii costume.
I hadn't heard anything about that leaker, thanks for clearing it up, I'm looking forward to that Arle Nadja Costume.