Neither one of us is right.
The legendary Reploid who fought valiantly and was known as a hero during the Maverick Wars. Though the soul is indeed that of Zero, the body that Ceil found him in was a copy of the original. Zero's original body, which had caused much damage during the Elf Wars, had been sent into space, and is now known as Omega. But Zero's new body, showing at least as much if not more potential than the last, has saved the world twice over from the evil intentions of Omega and Weil, the man controlling him. Zero was last seen in space. His current whereabouts unknown. Quote from Rockman Zero Complete Works page 24.
I am betting that not even Capcom knows what happened to Zero afterward (they're too lazy to come up with something). The last two sentences from the quote above basically has Capcom saying that no one knows for a fact what happened to Zero. Also this may be used to spark new theories by fans. The truth is that Capcom still is waiting to see if they are going to make a RMZ5 (if there are enough fan demands on it) or just keep it a mystery to further Zero's legacy.
Also according to this...
????: Biometal Report Classification #555913 * Chosen One * This message is
intended for the chosen one who will someday find the Biometal I have created.
In order to fight against Model W, which has been causing the Maverick
outbreaks, I have constructed Model X and Model Z with the data I have obtained
on ancient heroes. There is still so much I don't understand... And I was forced
to use the research on Model W as a basis for constructing the other two models.
In other words, the ones with the potential to use Model X and Z, are the same
one with the potential to unleash Model W. As a chosen one, you have the power
to save or destroy the world. I can only hope you use the power for good and
that you will decide to guide the people to a bright future. Quote from Rockman ZX
...what you say about Model Z containing Zero's soul (lack of a better word) is wrong.
Personal opinion: I never said Zero survived by being saved by someone (ex. Mother Elf, X, etc). He would have survived all by himself (beside the point so excuse that last statement).