He is the main antagonist of the Original Story.
It was revealed that before, he was a human named Xerxes, and had found one of Mew's hairs by accident. When he attempted to clone the creature from the hair, Mew sabotaged the project and caused the nuclear meltdown, leading to the creation of the Pokeumans. Xerxes was exposed to the mutant Mew DNA and was subsequently transformed into a Mewtwo, where he snapped and became evil.
WARNING!!! Plot and/or ending details below. Read carefully!It was also revealed that Asula is his sister, while she escaped being vaporized both were inflicted with the Beserk Gene which causes Mr. X's evil intentions and Asula's raging anger.
Spoilers end here!