As is explained in the original Mega Man X, he is the template on which all non-Light model Reploids (except Zero) are based, and he battles against Mavericks to protect humans and assuage his guilt for his part in their creation. He was created by Dr. Light sometime in 20XX. Light named him X after the variable "x" which, in algebraic terms, represents limitless possibility, as do X's advanced systems.

Light feared the ramifications of giving robots free will, so he chose to seal X inside a capsule for 30 years to test the integrity and reliability of X's systems.[1] Unfortunately, this landed well beyond Dr. Light's predicted life expectancy, and he died before X's diagnostics were complete. X was discovered in his capsule 100 years later by another scientist, Dr. Cain, who attempted to emulate X's technology and, with X's help, produced the first mass produced Reploids: humanoid androids based upon X's designs. When Reploids began to rebel against humanity (such rebels are termed Mavericks), the Maverick Hunters were formed to combat them, under the leadership of Sigma, the strongest and most advanced Reploid created at that time, as well as the first Reploid created by Cain. This proved to be disastrous when Sigma himself was eventually infected with the Maverick Virus, and went Maverick himself, taking most of the Maverick Hunters with him as he leads a massive revolt. X, feeling guilty for being the catalyst of the Maverick outbreak, takes it upon himself to help put a stop to Sigma and his plans, and teams up with Zero, one of the last remaining Hunters, in an attempt to find Sigma and put a stop to his revolt.