Mewtwo is a Pokémon created by science. It is a bipedal, humanoid creature with some feline features. It is primarily gray with a long, purple tail. On top of its head are two short, blunt horns, and it has purple eyes. A tube extends from the back of its skull to the top of its spine, bypassing its neck. It has a defined chest and shoulders, which resemble a breastplate. The three digits on each hand and foot have spherical tips. Its tail is thick at the base, but thins before ending in a small bulb.
Mega Mewtwo X is larger and bulkier than its normal state. The horns on its head are longer and extend from V-shaped ridges on its forehead. Additionally, the horns are now pointed and curve upward. It now has two tubes behind its neck, its eyes are light blue, and it has large, purple growths over its shoulders. Its torso is smooth and small compared to its long, bulky limbs, and a Y-shaped line connects the collar and underbelly. Its forearms and thighs each have two raised ridges and its digits are now longer and thinner. The tail is slightly shorter and stiffer than before and has a curled tip. Its muscles are augmented with psychic power, which increases its strength and sprinting abilities. When using moves in battle, it is shown to be able to stretch its arms and legs.
In comparison, Mega Mewtwo Y is smaller and lighter, losing its long tail and the tube behind its neck. However, it grows a long, purple appendage with a curled tip from the back of its head that is similar to its previous form's tail. On its head it has two pointed, backward-curving spikes and a half-ring structure connecting to base of each and crossing over the top of its head. Its eyes are now red and slightly larger. The digits on its hands and feet are now purple and more bulbous and the middle toe is longer than the others. A bumpy ridge similar to a sternum replaces the breastplate structure on its chest, but this form is still largely smoother than the other two. Despite its size, it has phenomenal mental power, and is capable of smashing buildings with a mere thought.
Mewtwo was created after years of horrific gene splicing and DNA engineering experiments based on Mew. It is said to have the most savage heart among all Pokémon, lack compassion, and strike fear into its enemy with cold, glowing eyes. Because it was engineered to be the ultimate in battle, it can only think of defeating its foes. In the first movie, where it was shown to be capable of levitation, telepathy and mind control, Mewtwo exemplified these vicious characteristics. However, it was later shown to be caring, protective, and even altruistic. Another Mewtwo in the anime shared its aversion to contact with others, but was much less belligerent.
Mewtwo conserves energy by remaining motionless in order to unleash its full power in battle. It is also capable of materializing psychic waves in the form of its signature move, Psystrike. Though rarely seen in the wild, it is said to be resting in a dark cave somewhere.
According to the scientific logs found in the Pokémon Mansion of Cinnabar Island, Mewtwo was born from a pregnant Mew, found deep in the jungles of Guyana, whose embryo had been tampered with to alter its DNA. It was held and studied in the mansion where a scientist performed horrific gene-splicing experiments that made it vicious and extremely powerful. It eventually broke free of the Mansion, destroying it in the process, and fled. It can be encountered in Cerulean Cave, accessible only after a Trainer has proven his or her skill at the Indigo Plateau.
It reappeared in Genesect and the Legend Awakened as both its normal form and Mega Mewtwo Y. At the beginning of the film, Mewtwo encountered the Genesect Army, who were looking for their home after being recreated. After a short conflict, the Army departed, and Mewtwo followed them to New Tork City. It met Ash and his friends, then got into a great conflict with the Red Genesect leading the Army.
Mewtwo appeared in File 4: Charizard. After finding out about its existence from Blue, Red entered Cerulean Cave to add Mewtwo's data to the Pokédex. In a fierce battle that wiped out almost all of Red's Pokémon, Mewtwo was finally defeated by Red's Charizard after it Mega Evolved into Mega Charizard X. Red then proceeded to catch Mewtwo, making his collection of the 150 known Pokémon complete.
Mewtwo has a similar origin and personality to the Mewtwo that appeared in the anime. It was created by Team Rocket scientist Blaine in Tauros the Tyrant, who added his own human DNA to Mewtwo to finish its creation when Mew's cells weren't enough. It was fully completed by the time of And Mewtwo Too?!. Together, Red and Blaine battled with Mewtwo, which eventually ended in its capture via Master Ball in And Mewtwo... Three!. It appeared again in the X & Y chapter, where it was seen sitting inside the Unknown Dungeon in Pokémon Village. It soon engaged in a battle against Lysandre's Pokémon and Mega Evolved into Mega Mewtwo Y. It was soon revealed that Mewtwo and Blaine had reunited, with their unique bond even allowing Mewtwo to Mega Evolve again into Mega Mewtwo X after switching its Mega Stone.
Pokémon Pinball: Mewtwo appears in its own bonus stage, where the player must attempt to hit it 25 times within the two-minute time limit to win. Playing this stage repeatedly is the fastest way to build a high score, due to the extremely high point output earned by hitting Mewtwo.
The island country of Niue released a one-dollar coin featuring Mewtwo as part of a commemorative promotion for the Pokémon franchise, with Mewtwo on the reverse side and the nation's crest on the obverse.
Mewtwo was originally depicted as the only member of its species, being a clone of Mew. In the anime and manga, however, it is implied that multiple Mewtwo have been created.
Mega Mewtwo Y was the first Mega Evolution revealed to the public and the first to appear in the anime, even before Mega Evolution was announced.
Prior to the announcement of Mega Evolution, Mega Mewtwo Y was referred to as its 「覚醒した姿」 Awakened Form. This term was also used in the manga adaptation of the movie.
Mewtwo's Mega Evolutions are tied with MegaRayquaza for the highest base stat total of all Pokémon, with 780.