The Rising Sea
After they had defeated Xemnas, Sora had arrived back at the Islands by falling into the water from the sky. Riku, too.
And while Kairi had never told anyone this... she'd had visions that Sora might do that to come back to her, before it had happened.
And now that she'd lost him in the most permanent way possible, she pondered if it might happen a second time.
But the thing was, Kairi was through waiting. And just thinking that she might have to do that again, had her balling her hand into a fist angrily, as she stood on the beach.
If Sora was destined to return to her via "falling into the sea" again. Kairi-as one named for the sea-would be sure to be anywhere and everywhere for him, so they could have their moment.
But she would remain stationary so longer.
Summoning her Keyblade, Kairi made a path towards the labs in Radiant Garden.
Author's Note: Because the KHI opening movie, the KHI secret ending, and the KHII ending all had Sora returning to Kairi by falling into the sea before her.