I've been wondering this for a long time now, and maybe I'm missing something in the X games, but who created Zero really?
I remember him first appearing in MMX, but there really didn't seem to be any indication of where he came from. I could almost say the same for X, but he most definately was modified and awakened with the new name X.
Zero must have been created from someone evil because of that whole Sigma Virus thing that happens in X5 and so on. And then maybe he was left with free will to decide for himself what he wanted to do. I'm very curious, so would anybody be able to expand in this area?
Zero was created by Wily and Wily tossed in the Zero Virus into him. It caused any reploid that was not created by Dr. Light or Dr. Wily. This is what happened to Sigma. Sigma was the first reploid to come into contact with Zero after Zero woke up. The virus most likely transfered to him. The virus spread only to "imperfect" reploids because Dr. Cain could not copy X completely.
Megaman Powerbatles 2, play as Bass and beat the game.
Wily speaks about how he's creating a new type of Robot which technology far more powerful than both Megaman and Bass and could easily wipe them out (Reploid). Wily implanted him also with the Zero virus.
Dr. Cain builds Sigma and other reploids to go against Zero and capture him. As sigma fights Zero, the Zero virus is tranferred from Zero himself into Sigma, and it evolves into a deadlier virus, the Sigma virus.
blah blah blah blah
Zero was created by Dr. Wily. That's fact - check the end of Power Battles 2, as Pit said, and most of X5. But what happened between then and when Zero awoke and fought Sigma is completely unknown - that is, there's no canon explanation for it. His virus was transfered to Sigma when he malfunctioned during their fight, but I'm a little foggy on details - whether it completely vanished from his systems in it's transfer to Sigma, or he was reprogrammed by Cain, I'm not sure (maybe a little of both?).
As for Sigma, he was supposed to be super-advanced (Maverick proof, *HINTHINT*), so he remained fairly sane until the start of X(1). Remember, he just walked past the two Reploids waiting outside, but was a lot more cranky than before. :) The virus (I'm still not sure if it was originally the Zero Virus, the same one that appears in X5, or just a virus) has essentially become part of Sigma, and vice-versa - proven by the fact that not only does that bald-headed creep keep coming BACK, but he frequently floats around as the virus.
X is most likely NOT MegaMan rebuilt. The most logical explanation is that his systems are WAY to different from MegaMan's for them to be one and the same, and besides that Dr. Light has shown a sort of father-son relationship with Mega, so it's unlikely Dr. Light would so completely change him (giving him battle abilities didn't change who "Rock" was, whereas turning him into X would be making him into a whole 'nother person).
X was discovered first. Why he was placed in the capsule is, as I said, unknown (Capcom hasn't giving much, if any, info on the subject). He HAD to have been discovered first, because the Reploids are based off his design (Reploids = replicates) and Sigma and all of Gamma Unit were there to fight Zero when he appeared.
I could go on forever, especially about the virus(es), but I think I covered the basics. :) If I missed something, please let me know. ^^