Mewtwo

"Fool, trying to stop our battle."

- Mewtwo

Mewtwo is a psychic genetic Pokemon cloned from Mew, the Legendary Pokemon ancestor of all Pokemons. Interestingly Nintendo considers Mewtwo the main antagonist of Pokemon. Mewtwo can be considered evil, but more misunderstood because it was the way he was created.

Mewtwo was created by Team Rocket by cloning DNA from a Mew fossil. (how they have the brains, we do not know.) It was Team Rocket's intention to create the most powerful Pokemon. However, Mewtwo was angered by the team as he heard that he was to be controlled by them, and in his fury, destroyed the lab where he was created with his intense psychic powers. Believing humans as oppressors of Pokemon, he vowed to exterminate humans and all Pokemon that sided with them, believing it to be his "purpose". He also wishes to exterminate Mew, as he believed he was vastly superior to his genetic counterpart. In the manga he was created during the Red, Blue, and Green story arc by the Gym Leader and Team Rocket's former scientist Blaine who used one of his own cells in the process though as a side effect whenever Mewtwo fights it causes Blaine immense pain due to the cellular connection with Blaine. Mewtwo first appeared attacking Blaine and it was only with Red's strategy that Mewtwo is caught in a Pokeball.

In the Yellow arc by this point he and Blaine have become good friends and fight alongside the Yellow against Lance of the Elite Four, but Blaine was forced to call him back before he died due to their connection, though Yellow with Red's Chu (Pikachu) would use the same strategy Mewtwo tried earlier to win.

In the Gold, Silver, and Crystal arc during the Gym Leader completion, thanks to the powers of Entei's holy flames, Mewtwo's connection to Blaine is severed to where Mewtwo can now fight without affecting his creator, allowing him to leave Cinnabar Island and be free.

In the FireRed and LeafGreen arc Mewtwo reappears and aids Red, Blue, and Green against Deoxys and Giovanni.

Trivia

 * Different from many villains, he has no sense of happiness and has no evil laugh, but at the end, you can see a tiny smile on his face.
 * The armor used to restrain Mewtwo in the FireRed and LeafGreen story arc of the Pokemon Adventures manga resembles the armor Mewtwo wore in the anime.
 * Mewtwo has a striking resemblance to Frieza from the Dragon Ball Z series.