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, and as such is able to learn any attack.
As demonstrated by its behavior in the first and eighth Pokémon movies, it shows signs of intelligence, curiosity, 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.
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.
A Mew made its physical main series debut in Alola, Kanto!, where it was seen flying around near Professor Oak's Laboratory. This is the latest main series physical debut of any of the original 151 Pokémon. It was also implied that Rotom attempted to hunt this Pokémon down to catalogue the Kanto Pokedex during the field trip, but could not find it by the time it returned to Professor Oak's lab.
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 Smash 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
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 a highest possible score of 10,000 points with perfect positioning, size and pose.
In PokéPark Wii: Pikachu's Adventure Mew takes Pikachu and their friends Chikorita, Piplup, and Charmander to the PokéPark. He then appears to Pikachu in a dream once they've arrived in the PokéPark and tells Pikachu to collect pieces of the Sky Prism so that peace can be restored to the PokéPark. Once all the pieces have been obtained Mew awaits Pikachu at the Sky Pavilion where he plays. In this game, Mew is referred to as male, despite being genderless in the main series.
Mew shares the move Transform with Ditto, along with also sharing the same Shiny coloration and the same weight. In addition, in Pokémon Yellow, Ditto can be encountered in the Pokémon Mansion on Cinnabar Island, where diary entries on Mew can be found.
Mew is similar to a vertebrate embryo. It shares many traits with cats. Its morphology is also vaguely similar to a jerboa, having large hind legs and a long skinny tail. It is believed by scientists to be the last universal common ancestor of all other Pokémon, which, along with its resemblance to an embryo, may be a reference to the recapitulation theory.
Moreover, Mew's design was based on Mewtwo. It was made simpler than Mewtwo's so that it would take up less space on the cartridge.
Mew may be derived from 妙 myō (strange/unusual), or mutant (myūtanto), or it may be a combination of mystery and new (considering that it is mysterious and "newly" discovered).