Speaking with IGN after today's D23 conference, Series Director Tetsuya Nomura revealed that Sora won't be the only playable character in Kingdom Hearts 3. While he declined to reveal who the mysterious person will be, it isn't the first time a game in the series featured another playable character aside from Sora:
"So it would be tough to introduce many, multiple playable characters, but there is an intention of adding a playable character aside from Sora. Unfortunately we’re not able to disclose who it is yet, but there will be [another playable character]," Nomura said.
Nomura also revealed that there may be more than 5 people in Sora's party at certain points in the game. Eagle eyed viewers will note that both Buzz Lightyear and Woody joined Sora in the Toy Story World, meaning they both joined Sora, Donald, and Goofy on their quest without getting sacrificing party members:
"D-Link, that has been done away with. It was actually a compromise in a previous iteration because we were not able to display multiple characters on that platform in that game," Nomura said. "We’re able to depict multiple characters. Up to five people can join, even more, multiple characters can join your party."