いつか本当の笑顔に。

(I was gonna reply, but then it started turning out really long so oops)

So the thing that people usually get wrong is that Type-0 isn't a multiverse. There is only one "world" and only one version of each character active at the same time. Each new cycle replaces the previous one, which is also why the order matters. It's a linear progression, so you can't just shuffle them around.

And by progression I mean that it's kind of like New Game+. People don't actually stay the same, that's the main point of Arecia's experiment. Everyone may lose their memories with each reincarnation, but their souls remember. That's how they grow and change, and why the events are slightly different each time.

For example, one of the writers describes that in the beginning of the experiment Class Zero were pretty weak and lacked a bunch of skills they have later on, like Ace was tone deaf, Seven kept getting tangled up in her own whip, or Jack could barely cut grass. It's essentially a similar concept to how people grow in real life. So someone at the age of 20 is at the 20th "cycle" of their life. But just as you can't go back to the 5-year-old version of you, neither can Orience denizens go back to the previous cycles. So if you open your Type-0 save file, you'll see the number of playthroughs on the right, but you can't replay any of the older ones unless you start from scratch. You can't delevel your characters or unlearn any of their abilities. That's essentially how the Spiral works.

There's a famous quote that goes: “Insanity is doing the same thing over and over and expecting different results.” And this is why Arecia isn't insane but just plain cruel. Anyone would go mad after fighting wars a million times, but if you take away their memories, they'll think it's their first time. Except it isn't, and every major event is etched onto their souls and directly affects their mental state in the next cycle. Which is what happens to Machina in the final cycle in the og Type-0.

But the interesting thing about soul memories is that everyone reacts differently. Cater's deja vu moments imply that she has a deeper connection with her past memories (or perhaps specific memories) than other characters. It could be related to personality, so Cater being an upbeat and no-nonsense kind of person may mean she's less keen on losing parts of herself.

It's kinda similar to how trauma makes you repress memories, but they're not actually gone. Like a certain Super Smash Bros. character would say, "Forgotten, but not lost." At the end of the Type-0 novel, Arecia shows Machina his memories of all the previous cycles. Which somehow doesn't break his mind, so it's technically possible to get all the past memories back if so desired. In the Agito novel, Naghi regains his memories of the past Finis after Tohno shows him hers. So you don't even need Arecia involved, just a potent enough trigger to "unlock" the memories.

At the end of Type-0, as seen in the True Ending, the souls cry out for release. Not just Class Zero's, but everyone else's as well. They've been through so much at that point that they no longer wish for any more do-overs, even at the cost of their bodies dying. People who assume that Arecia left Orience because she'd developed genuine feelings for Class Zero probably give her too much credit. She's essentially a God and has always been goal-oriented. She's shown on multiple occasions and even revealed herself that she sees Class Zero as tools, and this time the tools have finally outlived their usefulness.

(I have my own theories about Agito and where it fits, but I'll wrap it up here.)