Schala finds Serge and marries him, and Kid goes on to live her own life.
I like this explanation, as with everything discussed, it seems to fit the best. Only one question comes to my mind, though. Does Schala know/experience what Kid experiences in-game? As in, does Schala know Serge from the whole adventure, or does she just know that he's the one that saved her?
A
version of Schala finds and marries a
version of Serge, but we don't know anything else. It's not meant to be answered. We know that:
1/ Schala was saved by Serge and Kid during the game.
2/ Serge returns to Opassa Beach to meet Leena.
3/ Someone who signs as Schala Kid writes a diary and marries someone.
4/ Someone that looks like Kid looks for someone or something.
5/ Someone that looks like Schala Kid looks for someone in real life Tokyo and turns back to find you (the player).
Whether some or all of these characters are the same Schala Kid and Serge is unknown and not meant to be answered, as Kato clearly explained. In the end, Kato basically put the canon
aside for this ending and just wanted to illustrate the message that you too might meet your own Schala Kid one day.
It's just a magnified "soulmate" idea: the idea that you're destined to one day meet that one particular girl that you've always been looking for and that was always looking for "you". It isn't that far-removed from Xenogears' Contact and Antitype concept, except Xenogears' concept was pseudo-scientific and only applied to Fei and Elly whereas this is more Romantic and universal.