Created by Dr. Thomas Light, X is the upgraded version of his strongest creation, the original RockMan. He was developed in a way that allows him to think, feel and make his own decisions. He was sealed away by Dr. Light in an attempt to make him distinguish right from wrong; he believed that mankind was not ready to accept such an advanced robot.

Centuries later he was discovered by Dr. Cain, who used his outstanding design to build an entire race of robots capable of thinking for themselves: the Reploids.

However, these robots were defective, and started breaking the Laws of Robotics. This kind of "evil" Reploids were labeled Irregulars (or Mavericks), and soon a group of Irregular Hunters was gathered to mantain peace.

X joined the Irregular Hunters by himself, since he felt responsible for letting Dr. Cain create the defective Reploids.