It's kind of old already. And no, you can't use him.
His function is more or less like a Guess Character...
Don't know if this is worth making a thread over but I thought people should know about this. In two new screenshots it shows Kurasame as a playable character. I'm not sure if he's temporary for certain missions but it's still pretty neat. Sorry if this is old news, but I've never seen this anywhere yet.
I hope he's playable like the guest characters from FF12: International Zodiac Job System.
Well at any rate, it's good news that he's in the GAME DATA and has functional moves.
That would make it rather easy to implement him as a DLC or Multiplayer only character (As with KH358/2), and it also opens the door to other minor characters being available as guests, which is bound to improve the gameplay and optional quests.