Masahiro Sakurai explains the process of bringing Sora into Super Smash Bros. in his bi-weekly Famitsu Column. The column was given a rough translation on Twitter from game journalists @PushDustIn and @KodyNOKOLO.
According to the translations, neither Sakurai nor Nintendo was sure they would get Sora into Smash. Despite Sora being the number one voted character in the Smash Poll, the Fighters Pass Two DLC did not originally include him. It was only going to be five fighters, just like Fighters Pass One—Min Min, Steve, Sephiroth, Pyra/Mythra, and Kazuya were all already decided before the Smash team could get the go-ahead to bring Sora in.
By chance, Sakurai was able to meet with a Disney representative at a rewards show. It is there that Sakurai brought up wanting to get Sora into Smash. From there, Nintendo had long and careful talks with Square Enix and Disney. Eventually, Sakurai and his team got the okay and were allowed to put Sora in Smash. That's why Fighter Pass Two went from five characters to six.
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Sakurai's fateful encounter with the Disney representative echoes developer Tetsuya Nomura's meeting with a Disney rep in an elevator to get Kingdom Hearts created.
Another thing Sakurai talked about in the column was how complicated it was making Sora. The Smash team needed to follow many guidelines, and Disney and Square Enix supervised them. Creating DLC also took as much development time as working on a full game. In the end, Sakurai says he was glad to work on Ultimate and that the following column will be the end of his bi-weekly column in Famitsu.