"It's already gone back into the past."
God, please be just a bloated way of saying how Sora pressed the reset button after the Demon Tide moment and not another one of these "harr, harr, Sora has always been connected to the Age of Fairytales and of course this was foreshadowed ten years ago, but only came up now!" plot twists. Gameplay looks good though.
I don't think this is it.
I think, as other mentioned, chirity is talking about Kairi's heart.
Kairi had amnesia whe she fell on Destiny Islands, right? So, since that time, her heart was never reunited to her memories. Until Xehanort killed her and released her heart to The Final World. When Kairi's heart went to the final world, she then remembered who she really was. Her memories of being someone else finally returned, and then she goes back to her heart in the past, before she became Kairi.
Also, I was playing the game yesterday and I watched this scene again:
(1:18)
And I started theorizing a bit:
Kairi is "the girl", and the girl was Ava. which again Xigbar said "didn't make it, after all" in the secret ending. Well, Kairi lost her memories when she fell on Destiny Islands. But how, exactly? I think the answer is THE BLACK BOX. Ansem The Wise put her memories of being AVA inside the box.
I think it's weird how the phrasing of Ansem is "YOU HAVE SEEN THE GIRLS MEMORIES?", and "WHERE DID YOU PUT THE GIRL?" totally points to Kairi in the story. Like Ansem and Apprentice Xehanort, she is the only KH character who used to live in Radient garden (as seen in BBS) and was sent away when a kid.
In parallel to that, there is a Plot C happening about everyone looking for the black box. This is a far theory, but from what I can see, they expect her memories to be SOMEWHERE, like it's a tangible thing (and not just a normal memory loss).
The box probably has an important memory from Kairi when a kid in RG, or... has an important memory from ava.