He already messed with the natural order by going back in time once, and the natural laws of the world will more than likely destroy his own heart to keep him from going back in time and shattering the natural order again. However, he likely won’t be able to come back himself afterward. He’ll then have to reconstruct her heart right at the moment it was destroyed, and guide her back to the present. There, he meets Chirithy and learns that he can do so by going back in time again to the point where he prevented everyone’s deaths at the entrance to the Keyblade Graveyard, and by then using the Power of Awakening to enter his friends’ hearts at times before Kairi was killed. Set shortly after the end of the base game, the DLC shows Sora return to the Final World to find a way to restore Kairi’s heart.
After that, Kingdom Hearts 3’s ReMind DLC moves into its first playable portion.
