Mewtwo (ミュウツー), known as the "Genetic" species of Pokémon, is a Pokémon cloned from Mew that exists quietly in an undiscovered cave. It was created by scientists through genetic manipulation of Mew's DNA involving repeated recombination, gene splicing, and other DNA experiments over many years. While its DNA is mostly similar to the original Mew DNA, Mewtwo is very different in both its body size and personality. Its creators did not imbue him with compassion, making it one of the most savage Pokémon, able to strike fear into its enemies with its cold, glowing eyes. Being made for battle, its combat abilities and psychic powers were developed to be of the highest level, and it always thinks of defeating its opponents. In order to always fight at full power, it remains motionless to conserve energy.
In the events of the Pokémon: The First Movie, Mewtwo awakens, destroys the laboratory where it is created and being kept, and works under Giovanni, who had originally ordered Mewtwo's creation, until realizing it is just being used. After escaping, it tries to destroy the humans that 'enslave' Pokémon and start a new world. During Pokémon: Mewtwo Returns, it seeks complete solitude and chooses to live without fighting. Giovanni tries to recapture Mewtwo, but it is ultimately saved by Ash. Mewtwo chooses not to confine himself to the world and roams the earth. In the "Mastermind of the Mirage Pokémon" a mirage copy of Mewtwo is seen furiously battling Ash and its friends.
In Pokémon special, Mewtwo was created by Blaine based on Mew's DNA with some human cells from himself, during which an accident occurred and some of Mewtwo's cells backflowed into his arm, which responds every time Mewtwo uses its power, making it unable to fight for more than 3 minutes.
During the RGB series, Mewtwo made its first appearance in Team Rocket's laboratory under the Celadon Game Corner.
Later in the series, it escaped from the lab and hid near Cerulean Cave, where it was later captured by Blaine's Master Ball with Red's help.
During the Yellow series,
it was seen in Blaine's lab on Cinnabar island. Later, it was used by him against Lance where he attempted to destroy his pokéballs, which failed when Lance revealed that all of his pokémon were outside all along.
In the GSC series,
using Entei's flame, Blaine was finally released from the cells, and at the same time, Mewtwo left Cinnabar island.
In the FRLG series,
Mewtwo returned to fight alongside Red, being able to talk to him using telepathy.