Rettsu came from before the Shattering. Her planet was once whole, it is now in many disparate pieces which have their own orbit patterns in her solar system, this is what is called the Shattered Islands solar system.
Thanks to going through a wormhole is now thousands of years in the Future of that world or at least what Her world was. This would be devastating for anyone else but she made new friends rather quickly and her adventure brings her close to figuring out Why this all happens. By the time she reaches the end of the story, she realizes that it's time to close the loop.
Instead, she gave her Past Self her own Soul. This is sort of a New Game + in that definition...
There's twice the Rettsu, and she gets nicknamed Resettsu because of her exponentially more powerful Time Manipulation abilities. Her body has grown larger for whatever reason, and she can act independently of Dacutter and Lipupi since she's easily powerful enough by herself to rival any team.
Once Resettsu's story starts she gains the ability to interfere in the distant past. She can never go back to before her world was broken as she can't bring herself to see a world that was doomed, but she can more than certainly change what happened on the Islands in the three thousand years of history that they have had. And she starts by making Dacutter Island a better place.

I mostly just wanted to show off a newer-ish design of Rettsu that felt a bit like an upgrade.