I see now that the circumstances of one's birth are irrelevant; it is what you do with the gift of life that determines who you are.
It was created by a scientist after years of horrific gene splicing and DNA engineering experiments.
— Pokedex, Red and Blue
Mewtwo could be found in the first generation Pokemon games (Red, Green, Blue and Yellow) in the Cerulean Cave. Mewtwo was born of a Mew deep in the jungles of Guyana who's embryo was warped by genetic science. The young Mewtwo was held within a huge mansion on Cinnabar Island, where a series of cruel and disturbing scientific procedures were carried out on the young Pocket Monster, all under the oversight of the local Gym Leader Blaine and Mr. Fuji. Their experiments were all in the name of creating the most powerful Pokemon, and it seemed they had succeeded when the Mewtwo was finally able to retaliate, escaping from the Cinnabar Mansion, burning and destroying it with its incredible powers as it escaped. The mansion's destruction was a mystery to the majority of Cinnabar's citizens for years afterwards, but the young Trainer Red was able to uncover the truth from a series of Scientific Logs hidden throughout the Mansion's remnants before he continued on his journey to confront Blaine.
After Red defeated the Elite Four and became the champion, he was finally deemed worthy to enter the Cerulean Cave, a massive labyrinth filled to the brim with high leveled Pokemon. Unknown to Red, Mewtwo was awaiting him on the bottom level of the Cave- appearing for a single battle at level 70. Red could either defeat Mewtwo, who would then vanish and never cross the trainer's path again, or Red could capture it (most frequently with a Master Ball) and befriend the Legendary Pokemon, taking it on many adventures and battles.
A vicious psychic Pokémon created by genetic engineering. Its cold glowing eyes strike fear into the hearts of its enemies.
— Pokedex, Stadium
Mewtwo makes an appearance as the final boss of Pokemon: Stadium. After the completion of Stadium Mode and the Gym Leader Castle, Mewtwo's silhouette will appear in the sky over the Stadium for selection. The battle is a free-for-all with no rules- Trainers are encouraged to bring six Pokemon, either their own personal team or a group of Prime Cup Rentals, and battle a single Mewtwo. The Legendary Pokemon is level 100 and uses Psychic, Thunderbolt, Blizzard and Rest. After a difficult battle, the Trainer was able to successfully defeat the Mewtwo.
After the credits roll, the game allows for a second round against the Mewtwo. Round two makes Mewtwo even more powerful, boosting the damage done by its attacks, and replacing Blizzard with Amnesia, which sharply raises Mewtwo's special stat and increases protection against special attacks. Defeating Mewtwo again in the second round causes the game to replace the original title screen with a new one and unlock unique stickers in the gallery.
Because its battle abilities were raised to the ultimate level, it thinks only of defeating its foes.
— Pokedex, Gold
While Mewtwo does not make a natural appearance in the Johto games at any point, thanks to the Time Capsule feature that allows players to transfer their Pokemon from Red, Green, Blue and Yellow to Gold, Silver or Crystal, Mewtwo can be transferred into the second generation games. The only stipulation with this trade is that once Mewtwo is transferred, if it learns any new moves that were introduced in the second generation games, it can not be transferred back without using the Move Deleter first. Mewtwo could also be obtained through special Nintendo-sponsored events, creating the first time a player could potentially have two Mewtwo in a single party without trading with another player.
Mewtwo appears in Pokemon Stadium 2 as a member of Silver's party, alongside Lugia and Ho-Oh. After defeating the Stadium Cups and Gym Leader Castle, the Pokemon Trainer and Silver's rival Gold challenged his foe for a final showdown. Silver's Mewtwo has access to Psychic, ThunderPunch, Hidden Power and Safeguard. After clearing all of the challenges a second time, the second round with Silver is opened up and Gold can challenge him once again. This time, Mewtwo has access to Psychic, Thuderbolt, Hidden Power and Safeguard.
Mewtwo is a Pokémon that was created by genetic manipulation. However, even though the scientific power of humans created this Pokémon's body, they failed to endow Mewtwo with a compassionate heart.
— Pokedex, Ruby and Sapphire
Mewtwo was initially unavailable at all in the original Generation III trilogy- Ruby, Sapphire and Emerald, which instead focused on creating Legendary Pokemon of its own, and due to the change in hardware from the Gameboy Color to the Gameboy Advance, it became impossible to transfer from the past generation games. Not long after the release of these games however, a series of remakes of the original two first generation games were made- FireRed and LeafGreen. In these, Mewtwo could once again battle Red in the Cerulean Cave and could then be transferred to Ruby, Sapphire or Emerald. With a Gameboy Advanced to Gamecube connector, Mewtwo could also be transferred to Colosseum and XD: Gale of Darkness.
After the Pokemon Rescue Team was able to recruit the Legendary Ho-Oh from Mt. Faraway, they discovered the location of the legendary Mewtwo- the Western Cave. A 99 floor labyrinth designed like an even more grueling Cerulean Cave, Mewtwo had transformed the cave into one of the most dangerous locations for Pokemon on the globe. After fighting through 99 floors of the most dangerous Pokemon from the Kanto, Johto and Hoenn regions, the Rescue Team finally confronted the Legendary Psychic on the 99th floor. After a long fought battle, Mewtwo was defeated and decided to join the ranks of the team, staying in the Cryptic Cave when not on missions.
Mewtwo is definitely not a speedy character, but its ESP-powered grab and throw moves are comparatively strong.
— Mewtwo Trophy
Mewtwo is a secret character in the second Smash Brothers game, unlocked after playing 20 cumulative hours of Vs. Mode or from clearing 700 Vs. Mode matches. Mewtwo's playstyle in the game is incredibly unique, and due to a harsh learning curve, has been historically thought of as a bottom-tier character. Mewtwo utilizes a very floaty style, and uses a variety of combos and strong throws for offense. Mewtwo utilizes excellent wavedashing, phenomenal recovery, strong projectiles and above average dodges to dominate foes, but the trade off is that Mewtwo is a fairly light character with a large size, making it a victim of harsh combos, a weakness only further emphasized by his float. Mewtwo also has a slow basic movement speed, and lacks a decent 'KO' move to go to for ending bread and butter combos, making it difficult to knock out other players, even after dealing extensive damage. Mewtwo's neutral special attack is Shadow Ball- a powerful ball of energy that is as difficult to avoid as it is to aim, and can be charged up for more damage and greater knock-back. The side special attack, Confusion, is a high-risk but low-reward move that causes an enemy very near to it to fall over, allowing for potential combo set-ups though the long lag at the end of the attack often negates this. Confusion can also redirect projectiles, but these projectiles can not harm the person who originally fired them. Mewtwo's up special move, Teleport, allows it to escape an attack or projectile by disappearing and reappearing in the direction pushed by the player- though if Mewtwo teleports into the air, it will fall down in a helpless state until it lands. Mewtwo's down special, Disable, is a short-range projectile that stops an opponents attack animation and knocks them back, though it is often considered impractical for any serious play.