Who's that "Mii costume guy" who mentioned Byleth?
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.