Mew is a pink, bipedal Pokémon with mammalian features. It has a rounded, wide snout; triangular ears; and large, blue eyes. It has short arms with three-fingered paws and large hind paws with oval markings on the soles. Its tail is long and thin with an ovoid tip. Its fur is so fine and thin, it can only be seen under a microscope. Mew is said to have the DNA of every single Pokémon contained within its body, allowing Mew to learn and use any attack.
As demonstrated by its behavior in the first and eighth Pokémon movies, it shows signs of intelligence, curiosity, shyness, playfulness, and even selflessness. Mew is incredibly adaptable, able to travel freely in the air or underwater. As seen in Pokémon Snap and the Super Smash Bros. series, it can create a green, yellow, or pink orb of energy around itself for protection, although only the pink orb is seen in the latter.
According to the radio drama, The Birth of Mewtwo, Mew was discovered by Jessie's mother, Miyamoto, who registered its cry in Guayana Forest 20 years before the events of Generation I. It was worshiped for its protection by indigenous people who created a folk song in its honor and believed it appeared once a year with the rising sun. It wasn't rediscovered until 18 years after Miyamoto met Mew by sunrise on the peak of the highest mountain in a South American mountain range and fell.
Reports found in Cinnabar Island's Pokémon Mansion note scientific expeditions that have sighted Mew in Guyana, South America, the place where it was first discovered. Since Mew can make itself invisible at will, very few people have knowingly seen it, leading some scientists to declare it extinct and most to assume it to be a mirage. It will only show itself to a person who is pure of heart and has a strong desire to see it for themselves.
Mew first appeared in a flashback in Enter Pikachu!, four years prior to the events of Pokémon - I Choose You!. Goh and Chloe spotted it while attending Professor Oak's Summer Camp, where it was seen battling a Nidoking. Mew soon ran off, leading Goh and Chloe to chase it. Mew saved a baby Kangaskhan from a fall before leaving. After reporting this to Professor Oak, Goh became determined to make Mew his first capture.
In Super Smash Bros., Mew will sometimes appear out of a Poké Ball. When summoned, it will use Fly to leave the arena. In single-player, the summoner is awarded the "Mew Catcher" bonus, worth 10,000 points.
After unlocking every character in Super Smash Bros. Melee, there is a 1 in 251 chance of Mew appearing from a Poké Ball. The thrower of the Poké Ball receives the same "Mew Catcher" bonus.
Mew returns again as a rare Pokémon in Super Smash Bros. Brawl, the chance of it appearing is 1 in 493 this time. Like the other "secret" Pokémon, it drops rare items instead of just flying away. In Mew's case, the items are CDs. However, once all the pick-up CDs are obtained, it will drop Stickers instead.
Throughout the series, Mew, much like Mewtwo, is roughly akin to its portrayal in the first movie, especially having Kōichi Yamadera reprise his role.
Melee trophy information
Mew is an extremely rare Pokémon that has been seen by only a few people. Information on every Pokémon in the world is contained in Mew's cell structure, so it has the ability to use any and all TMs and HMs. Some Pokémon scholars believe Mew to be the ancestor of all existing Pokémon, but the idea is debatable.
Brawl trophy information
"A New Species Pokémon. An extremely rare Pokémon that contains the DNA of all other Pokémon, it is thought by many scholars to be a Pokémon ancestor, considering the wide range of moves it can use. On top of being able to make itself invisible, Mew can also transform into any opponent Pokémon and use characteristics and moves in exactly the same way."
3DS/Wii U trophy information
Mew trophy in Super Smash Bros. for 3DS
Mew appears as a trophy in both versions of the game.
NA: Mew is a Pokémon previously thought to be extinct but rediscovered by Dr. Fuji. This Legendary Pokémon doesn't often show itself to humans, so it is considered quite lucky to see one. In Smash Bros., Mew will appear and then use Fly to sail away, leaving behind a gift. Thanks, Mew!
PAL: Mew was thought to be extinct until it was rediscovered by Dr. Fuji in Guyana. This Mythical Pokémon doesn't often show itself to humans, so it's considered very lucky to see one. In this game, it will float upwards and leave a present behind. If only Mew would turn up more often, eh?
Mew appears in the seventh course, Rainbow Cloud. Although the game is not combat-oriented, the difficulty of photographing Mew effectively makes it the game's boss. Mew floats in a bubble that turns opaque when the zoom is activated, so players must throw a series of Pester Balls or Food at it to break the bubble so Mew will come out for a clear shot. Photographing Mew gets an automatic bonus of 2500 points, making it the highest-ranking Pokémon in the game with the highest possible score of 10,000 points with perfect positioning, size, and pose.