...millions of years ago, a new step in science was being taken by a species that was technologiacally superior to anything else that lived in the universe. It was a time when life was still primitive on Earth, and was only advanced on this single planet, far from everything. A ship was created, when one of the tribes began what was called, the "Absolute Zero" project. The tribe set out to create the perfect artificial being. One so powerful, that all of the rival tribes put together could not stop it with all their armies, and all of their weapons. The project was a dangerous one, and was taken on its ship, far out into space, as far from its home planet as possible. On the ship, advanced humaniod robotics called "Repliroids" were created. There were 20 in all, and each one was a different from the others. They were all based on different theories of what the "Perfect Repliroid" would be. Once finished, they had to be activated under stasis, where they would be kept for 30 years. If they were let out before 30 years, their programming would be ignored by their consciousness, and they would have no behavioral limits. In such a state, they were far too dangerous.

Among them were Xero Unit-00, and the "Repliroid" that was X's female form.

Also among them were the designs for a "Repliroid" called "X". Only the designs were there; there were no prototypes for this machine. It's design was one of the most potentially dangerous. "X", being a changing and/or unknown variable was an appropriate name for the "Repliroid" who's compatability with ANYTHING was full, and limitless. It was compatible with any armor, and was able to gather high amounts of energy just on the very tip of it's powerful Buster Gun, so it wouldnt overload or overheat. "X" was the greatest design yet created, and the project was starting to look

The parts for the first "X" prototype were about to be built, but a terrible disaster overtook the ship. The crew was killed, and the ship crash-landed on a small planet. Earth. The reploids were kept in stasis for too long, and died when the power in the ship ran out. All except Xero, because of his regenerative abilities. Xero, disoriented and hit with severe amnesia, was found by scientists who were investigating an energy reading coming from the island. He was revived from his state of near death, and recruited for a mission.

Long before those scientists found the island, however, the designs for "X" were found by a single scientist, and modified so he would be possible to build with today's technology.