Well, I did a lot of digging, and hoo boy SMRPG's legal rights are potentially a fustercluck. Because of how Square (and S-E) handles copyright, we have wonky situations like FFVII's character rights (Amano has the rights to his artwork and Uetmatsu to his music, for example - which is probably why Cloud has original artwork in his Fighter Spirit) and Sugiyama being extremely stingy about his DQ music (the MIDI thing). There are
4 character designers listed in SMRPGs credits. While we don't know exactly who has the rights to Geno, I researched all 4 - quickly at that - just to be safe (and I'm linking to each designer's MobyGames page - with the exception being Kotabe since he was a Nintendo stalwart):
Kotabe and Hatae are likely not any problems at all - Hatae still works at S-E (he's involved with KH3). Now, Kato and Kurashima on the other hand...
Oh dear. Kurashima was involved in the M&L cameo (he's listed there as "Mini-games by Vanpool"), but he left Vanpool in 2009 to become a freelancer. He eventually joined
Onion Games and works there currently. His latest project was helping to port the PS1 RPG
Moon Remix RPG Adventure (which he had worked on back in 1997) to the Switch. I'm also bringing up Kurashima because he could have the rights to Smithy (if he's classified as a Monster) and other antagonists - potentially making a Spirit Board more expensive if it turns out that Kurashima doesn't have (exclusive) rights to Geno. Since the upcoming DLC characters are "Challenger Packs" I presume they'll have DLC Spirit boards like the FP1 characters did.
Out of the character designers it seems like Kurashima might be the best guess - he has a
Twitter account, an
Instagram account, and (a seemingly defunct) page on S-E's website. The others don't turn much up (after just cursory research admittedly).
Kato became much less active after Tactics and likely retired after 2011. He certainly doesn't seem to have left much of a trace.
I also stumbled upon
Taro Kudo, one of SMRPG's event designers (
Keisuke Matsuhara and
Yasushi Matsumura being the other ones): Kudo was also involved in the M&L cameo. However, he is also involved in Smash 4 / Ultimate - as Dillon's voice (something he also does at Vanpool).
He's the only one of the seven I've mentioned here to have a credit in Sm4sh and Ultimate. He was also involved with Moon Remix / Vanpool together with Kurashima for 12 years, and helped develop Sticker Star / Color Splash. He's still at Vanpool - with his latest credit being Dillon's Dead-Heat Breakers.
I even went and checked Dillion's voice clips in 4 and Ultimate - Ultimate reused 4's voiceclips (a common theme with voiceclips since Nintendo owns those as part of the game). But still:
How did Square divvy up the SMRPG cast's rights? Like who has Boshi's rights? DQ's Spirits used enemies to fill out the numbers (and Luminary's party is one Spirit) - is this possible with SMRPG's characters without running into budget problems? Also, is it just Kurashima that Sakurai and Nintendo need(ed) to talk to, or do they also need to talk to Hatae / any others? How did - and does - Kudo / Kurashima view Geno?
Like, if Kudo was with Nintendo since 2011ish - and he's both involved in the M&L cameo and in Sm4sh / Ultimate,
and what we've seen so far are a Mii Costume and two Spirits - then that just raises even more questions and / or weird answers.
All in all, my position on Geno is and will be "I believe it when I see it". Especially when it looks like Geno's legal position is "Fustercluck" territory.