The 1st and 2nd escapees from Celestia; because of that, Arison decided to give them infinite lives, so there could always be a Celestia living among normal human beings.

Arison didn't know how many escapees there would be in total, so it made a safe bet for them to always stay among the living, not in one infinite life, but rather in never-ending "universes" named rotations. Every time 1 and 2 died, they were reborn into a new rotation, not remembering their old lives or names. They only vaguely remember each other, but those memories come between the ages of 16-21. It isn't a sudden realization; it's a slow, confusing process. Also, at that age they develop special marks that can be used to identify each other, which, to a normal human, look like tattoos.

In the story "xxx", they are in rotation 270,214.