First post where I talk about two "rejected" indie characters and multiple people can confirm one is Cuphead. The other is probably coming in Pack 6 if I had to guess.
This is I believe the first post where I talk about people knowing about Undertale content in Ultimate as early as May.
Just to get people up to speed, the following names were rumored to be "rejected" by the development team according to different sources since November but many wind up being Mii costumes in the end:
Elecman, Ms. Pac-Man, Gooigi, Cuphead (confirmed), KOS-MOS, Travis, Nightmare, Ryu, "a Microsoft character asked to be kept secret", "a Square Enix character asked to be kept secret", Arle, Kris (Deltarune), Knight (Hollow Knight), "an indie asked to be kept secret", and "a character that no hints want to be given for at all."