Mewtwo is an artificial Pokémon. It is a bipedal, humanoid Pokémon with some feline features. Its body is primarily gray with a long, purple tail and a large purple underbelly. 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.

According to journals found in Kanto's Pokémon Mansion, after discovering the Mythical Pokémon and gathering some of its DNA, Mr. Fuji used it in his mansion to create Mewtwo, a modified clone of Mew with the goal to making it the most powerful Pokémon of all. It was held and studied in the mansion where horrific gene-splicing experiments were made making the Pokémon vicious and extremely powerful. It eventually broke free and fled the Mansion, destroying it in the process. It is widely known as one of the most powerful Pokémon in existence and is said to have the most savage heart among all Pokémon, lacking compassion and striking fear into its enemy with cold, glowing eyes. Mewtwo conserves energy by remaining motionless in order to unleash its full power in battle. Because it was engineered to be the ultimate opponent in battle, it can only think of defeating its foes. Though rarely seen in the wild, it is said to be resting in a dark cave somewhere.

In Mewtwo Strikes Back (M01), a Mewtwo exemplified its vicious characteristics. It was shown to be capable of levitation, telepathy, mind control, memory erasure, teleporting others to another location, and the creation of powerful storms. However, it was later shown to be caring, protective, and even altruistic. Another Mewtwo in Genesect and the Legend Awakened (M16) also shared its aversion to contact with others but was much less belligerent. As shown in POKÉMON Detective Pikachu, Mewtwo can also transfer a human's soul into a Pokémon's body provided that said Pokémon is in a crazed state, causing this person to gain control of the Pokémon's body and abilities. It possesses a rivalry with Mew. As seen in Genesect and the Legend Awakened, Mewtwo can breathe in outer space. In the Pokémon Adventures manga, it's revealed Mewtwo's creation wasn't possible with the amount of Mew's cells gathered, so to complete it, human cells were used instead, specifically from the Team Rocket scientist Blaine. Due to Blaine possessing some of Mewtwo's cells as well, the two became linked, which granted Blaine to ability to track its whereabouts at the cost of slowly taking his life.

Shadow Mewtwo
A unique variant of Mewtwo, called Shadow Mewtwo, is very similar to standard Mewtwo, but with dark gray skin, a dark gray tail with a yellowish orange tip, and a shard of Shadow Synergy Stone coming out of its shoulder. In the past, Anne was almost possessed by a Shadow Synergy Stone but was saved by Mewtwo, whom the Shadow Synergy Stone took control of and merged with instead. Under the stone's influence, Shadow Mewtwo travelled around Ferrum, draining Gaia Power from other normal Synergy Stones, which continuously increases its power.

Forms
Mewtwo has two forms. Mewtwo can Mega Evolve into either Mega Mewtwo X or Mega Mewtwo Y. Additionally, Shadow Mewtwo can Mega Evolve into Mega Shadow Mewtwo X by using Synergy Burst.

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. Its tail is slightly shorter and stiffer than before and has a curled tip. Its muscles are augmented with psychic power, giving it a grip strength of one ton and the ability to sprint 100 meters in two seconds. When using moves in battle, it is shown to be able to stretch its arms and legs.

Mega Shadow Mewtwo X greatly resembles its normal counterpart, though it maintains its dark gray coloring due to the influence of the Shadow Synergy Stone. The stone's infection spreads not only to its tail, but to both of its shoulder growths, its arm ridges, and the tips of its ears. Additionally, the Y-shaped line running across its body (now including its legs) glows the same yellowish-orange color.

Mega Mewtwo Y is smaller and lighter, losing its long tail, underbelly, 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 and also slightly resembles Jessie's hairstyle. On its head, it has two pointed, backward-curving spikes and a half-ring structure connecting to the 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 decreased size, it has phenomenal mental power and is capable of smashing buildings with a mere thought.

(Source: Bulbapedia)