In a recent interview with IGN, Kingdom Hearts III Director Tetsuya Nomura teased that there could be a second playable character in Kingdom Hearts III. Nomura stated..
“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].”
Some have speculated that it could be Riku or Kairi as they both acquire keyblades at the end of Kingdom Hearts HD 2.8 Final Chapter Prologue