Everyone besides data Sora and Data Riku forgets things the next day, so Selphie, Wakka, Tidus, Kairi, all lose their memories after each day and I can't wait for them to interact with the real version of some characters.
The reason they lose their memories is because the journal resets so things are how it is supposed to be, "in a perfect state" so nothing can crossover or change. The only reason Kairi really is there is because Data Riku made her since she is "his" —quotations because he doesn't see himself as Riku but he uses the name.—and Sora's friend. Data Riku could technically try to figure out how to make them all remember but that would mess with the journal itself again. However, Sora finds her and the rest of them boring because they never remember which is why he prefers to spend his time with Data Riku or talking to the real versions.
Data Riku I mentioned sees himself as someone separate from Riku. He uses Riku's appearance and at first he was less like a human within the game and after time he becomes more like Riku and more human-like. He uses Riku's name but he knows he's not THE RIKU™. He knows he looks like him and is similar and he is supposed to be him in a way that his role is to be "Riku" but he's not him.
Also he'll do anything for Sora since he ended up loving him because he wants to but not because Riku necessarily does. The real Riku and Sora obviously care for each other but they run in circles when it comes to their feelings.
Data Riku now ends up loving Data Sora and the two of them are much more simple in that matter. Data Riku chooses to love and care for him because he ends up cherishing Sora and he becomes very important to him as he actually gets to know this data Sora. It's much more simple because they are different.
Now Sora on the other hand sees himself as Sora and notes that they're similar. They, however, are different because Data Sora can handle hurt where Sora can't. Sora is more worried and has more fears whereas Data Sora doesn't really worry as much. He'll worry if Data Riku will be okay if he gets hurt but that's basically it. Sora will worry if Riku still likes him and if he will hurt him like he did in kh1 Hollow Bastion because he has trauma from being left alone.
Castle Oblivion things actually did happen. Now how would that work since the big thing is he doesn't have memories. Well when Data Sora is in Castle Oblivion he doesn't remember but once he gets to the last floor, he remembers everything again especially after talking to Naminé.
Data Sora is how everyone thinks Sora is along with my own little traits. Like I think Data Sora (and even Data Riku too) are more excited and hyper about various situations. He can be very blunt and says what he wants. For instance when he saw the other Riku, he thought he was pretty and asked his Riku if he could have hair like that. He often frequently talks about Data Riku and mentions how he is his boyfriend because he loves him so much.
This all is based off my fic series The Reconnected Data