A five-page ramble, condensed to one. I still feel self-conscious of the length enough to use a cut, though.
Some world-building facts:
Data worlds have two modes: theatre mode and free mode.
All true data-people who have Minds (the data equivalent of Hearts) and are developing AI have one world which is their homeworld. They are considered “citizens” of that world.
Only true citizens of a world can access free mode of that world. Consequently, citizens are confined to their homeworld in free mode. They may still appear in theatre mode of other worlds, though.
Exception: Data-people who have been manually upgraded to Program status by Users or other Programs can go wherever they please. Including the spaces between and underneath data worlds, such as Backdoor Areas, System Sectors, and the Central Hub.
Exception #2 is the world that acts as the Datascape’s recycle bin. Anyone can get chucked in there.
Data-Sora is a citizen of the Destiny Islands. This is why he was able to chill there between canon and this AU’s first adventure (when he gets modded to Program status and becomes SO) without being stuck in a theatre mode loop.
Generally, Nobodies are not recognized by the Datascape as true data-people. They are not citizens of anywhere, and have no Minds. They can never access any free mode, and don’t grow AI. They exist solely as theatre mode actors.
The exceptions, of course, are Namine and Roxas. For all the usual “they’re special” reasons. They are citizens of Castle Oblivion, but that world is still corrupted so they didn’t actually have a free mode. Data-Namine was upgraded to NAMI almost immediately after being recovered from the recycle bin world though, freeing her to go wherever. Data-Roxas was basically just dormant until someone thought to go look for him.
The World That Never Was appears because it is discussed in the KH2 Journal. However, it is the Nobody equivalent of a world. No one is a citizen of it. Programs can access a free mode, but nothing ever happens there aside from the occasional appearance of bug blox, monstery nobodies, and heartless due to glitches. It’s creepy and no one likes going there.
Okay these are getting to big for comfortable bullet points.
The Journal & Datascape know all, even when they don’t consciously know that they know. Secret feelings, intentions, and memories seep into written accounts from their authors, and get extracted to the Datascape as Hidden Data. The process for discovering and recovering Hidden Data is different every time. The true data-person versions of Namine, Roxas, Lea, Xion, Aqua, Ventus, Terra, and Repliku (because fuck the writers for acting like he never happened!! HE COUNTS AS A HURT, DAMNIT.) are all Hidden Data. They have been observed by data-Sora and User Mickey, but so far only Namine and Roxas have been properly recovered.
All this is why AXEL is a wall/hologram. He is a glitch. Data from the KH2 Journal and the Dream Journals became entangled and tried to overwrite itself, due to reading data-Axel and data-Lea as the same entity. The unextractable nature of data-Axel corrupted data-Lea and caused their fused form to only be semi-extractable. Additionally, the fact that Lea is originally from Radiant Garden became Hidden Data when Lea uploaded his Dream Journal. So in theory, Lea’s body COULD be extracted from Radiant Garden eventually, but the data revealing his presence there is Hidden. Therefore his model is Hidden. So for now, AXEL exists as a wall between the Datascape render and the Void. Though his hologram can wander around the cytoplasm-esque Backdoor Areas between actual worlds and... himself.