Mew & Mewtwo
Pokémon begins and ends with Mew. The name of Mew was the first that Game Freak trademarked, and they designed Mew as the ancient, common ancestor of all the hundreds of other Pokémon. Mew is flexible enough to learn any move or become anything, strong enough to live forever, cute enough to win a place in our hearts. Mew is a phantom, worshipped as a protective spirit; a cryptid, sought by only the most laughable of scientists and treasure-seekers; a mirage, encountered only by extreme chance (and perhaps a few programming glitches). This Pokémon was the last Pokémon programmed into the original games, the final, mysterious 151st entry in original list of 150 Pokémon, the ultimate challenge in truly completing a PokéDex. Those who seek power, fame, or fortune, those desperate with nothing left to lose, seek Mew in hopes of capturing such pure power and unravelling untold secrets.
In the absence of the genuine article, however, one can settle for the best replica that science can produce. Perhaps humanity, in its pride, might even be able to create a creature stronger and more ruthless than the gentle guardian who is more likely to turn invisible and play tricks than wield its unimaginable psychic powers. Knowledge of genetics can be a dangerous thing in the hands of those with too much ambition and too few moral scruples...
Mewtwo, as the name suggests, is a clone of Mew, created with no purpose except to serve as an experiment and a slave, a purpose which he rejected. Mewtwo struggles, often violently, with his condition as the mere shadow of Mew and searches for a new purpose for his unnatural existence, to justify his right to be alive. He is the Frankenstein's monster of the Pokémon world, a mutated form with only a vague resemblance to the source of his DNA, an untameable, brutal force, and the final challenge for any Pokémon Masters who dare to stumble upon his hiding place.