Vergeben is legit. That much is more or less without question. He's been told multiple times by multiple people that a Square Enix character is going to be in. Now I'm not sure exactly what role at the company would allow you to have that information without knowing who specifically the character is, but he's been told it enough times that he is certain it's true. He's also said that if he said the name of the character he's heard from one source and he turned out to be wrong, there'd be hell to pay. I can think of no other character with that kind of staying power and fanbase.
Let's go into this Square Enix character concept a bit further since it leads us to some interesting outcomes. An important fact to remember going in to Ultimate is that
Sakurai already licensed Geno from Square Enix. A character's likeness in the form of a costume (an incredibly detailed costume a la K. Rool, I might add) is legally the exact same thing as the character itself. Permission has already been granted to use the character.
Now let's expand on this with some logical deductions. A Square Enix character is in. Sakurai already has permission to use Geno. Realistically speaking, it would be more of a bureaucratic difficulty to put in a SE character that
wasn't Geno. They'd have to go through yet another round of negotiations and shelling out money for permission to use whatever character that was, and it's pretty clear from the content we've seen that all of the licenses SE granted Nintendo in Smash 4 have carried over to Ultimate. We've got Cloud. We've got Midgar. We've got both songs. If there were
renegotiations, I sincerely doubt we'd only have the same two songs. It's pretty clear to me that whatever agreement Nintendo and Square Enix had applied to both games.
While we're on the subject of music, Nintendo used "Beware the Forest's Mushrooms" during the
Mii costume reveal. This is a song that would have required licensing, all for a 10 second video clip?
When you combine this with the comments Sakurai has made before, saying very directly that he's wanted to put Geno in the game for over a decade now, I think it would actually be offensive to not put him in. He knows the popularity of the character. We've gotten Ridley and K. Rool. The only logical conclusion I can come to is Geno being in as well.
I'm not even a longtime Geno supporter. I've been around Smash speculation since the early Brawl days so I have a bit of a soft spot for him, but I'm mostly just recognizing what makes the most sense. It'd almost be poetic at this point to
finally see him join the battle.