Mewtwo

Mewtwo is a Psychic-type Legendary Pokémon introduced in the Generation I games Pocket Monsters Red and Green Versions (released internationally as Pokémon Red and Blue Versions). It was created artificially based on the genetic code of Mew to be the ultimate battling Pokémon, but it grew too vicious and powerful to be controlled and escaped. Mewtwo has the most violent nature compared to several Pokémon. As of Pokémon X and Y, Mewtwo can Mega Evolve into either Mega Mewtwo X or Mega Mewtwo Y.

Aside from the core series, Mewtwo is the main focus of Pokémon the First Movie: Mewtwo vs. Mew, along with Mew, and it is a playable fighter in a few of the Super Smash Bros. series titles.

Pokémon Red and Blue Versions
In Pokémon Red and Blue Versions, the player character first learns of Mewtwo by reading research documents left in a ruined laboratory on Cinnabar Island. Mewtwo has the highest stats of any Pokémon.

Quotes

 * "Watashi wa naze koko ni iru no ka...? (Why am I here...?)" - Super Smash Bros. Melee, Super Smash Bros. for Nintendo 3DS / Wii U, Super Smash Bros. Ultimate
 * "Oroka na...! (Foolish...!)" - Super Smash Bros. Melee, Super Smash Bros. for Nintendo 3DS / Wii U, Super Smash Bros. Ultimate