Okay. I got two "out there" guesses.
One is this. Rhythm Heaven is not as first party as we think. The rights to it are co-held by TNX to the point where in the credits to Rhythm Heaven Fever, it says "Nintendo and TNX are the authors of this software for the purpose of copyright."
https://www.youtube.com/watch?v=WNnMbqQQ8tA
If you want to look deeply, maybe the three fingers is referring to the Chorus Kids? Or another Rhythm Heaven representative?
As for the other "out there" guess, remember that gun Sakurai bought for modelling for Ultimate?
https://gonintendo.com/stories/2711...cent-mech-toy-purchase-he-made-teases-that-it
Another "kind of first party" character, would be this character owned by HAL Laboratories.
That is V-MH GLORY from Metal Slader Glory, a text adventure developed by HAL Laboratories for the Famicom back in 1991, and remade for the Super Famicom in 2000 (yes, that late in the console's life). Sakurai seems to be almost priding himself on the character being something that most people will not guess, so maybe this is it, if it truly is an "out there" character, this one is really one of those.
Art of V-MH Glory:
This is the kind of character where you could know absolutely nothing about it, but still want to play as it due to the design alone.
We shall see who it ends up being. It really could be anyone.
However, if the info of the character being "sort of first-party" is legit, then these are my two guesses.