Sorry, I hope no one here is against risking being spoiled, or having spoilers, I'm just so excited about saying this. Okay, in Kingdom Hearts III storyline, it is said that Sora will be searching for "the key that returns hearts". Now that I am typing this, I am wondering something. Could this be how Xehanort, Braig, Dilan, Even, Aeleus, Ienzo Isa, & Lea came back? Let me know if I am putting something that was already posted. And another thing, I wonder what the could have to do with the Keyblade war? I know I'm curious, but, some things, I just have to find out. Also, when asked if Riku & Kairi would be playable, Tetsuya Nomura said that he "couldn't deny the possibility".

No, they were reconstructed/recompleted through the elimination of both their Heartlesses and Nobodies.  Yen Sid and Ienzo explain this, when Mickey and Lea respectively wonder about it, in enough detail.

Edited August 21, 201312 yr by Jaune Arc

I hope Kairi is the character we play in the tutorial.