Masahiro Sakurai disclosed details in his Famitsu column about the process of getting Sora into Super Smash Bros. Ultimate. While the quote doesn’t go into any great detail, it reveals that Sora’s inclusion happened by chance. Having been the most requested character among Super Smash Bros. players, there were several hurdles the team had to overcome to get Sora into the game. [Thanks, ryokutya2089!]
Sakurai mentions in his column that there was an incredibly high barrier that the team had to overcome to put Sora into Super Smash Bros. Ultimate. This was largely due to having to negotiate with Disney. Please keep in mind that the following translation is taken from an excerpt from the full column. Sakurai goes a bit into detail, saying:
There was no doubt that the team involved in the external negotiations thought this would be impossible. But one day, an opportunity appeared. At a certain award venue, I had the chance to meet a Disney executive. I told them that I would like to see Sora in Super Smash Bros. Ultimate.. There were a lot of negotiations between Disney, Square Enix, and Nintendo.”
Apparently, due to this chance meeting Sakurai was able to express interest in seeing Sora in Super Smash Bros. Ultimate with a Disney representative. This allowed for negotiations to continue between the three separate corporations, which would eventually see the Kingdom Hearts protagonist added into the title as the final Smash fighter.
Super Smash Bros. Ultimate is immediately available on the Nintendo Switch.