Celebi

Celebi (セレビィ Celebi) is a powerful /-type Mythical Pokémon first introduced in Generation II. It was the last Pokémon in the National Pokédex until Generation III was released. It can only be caught in Pokemon Crystal by using the GS Ball, which can only be gotten in an event.

Appearance
Celebi is a green Pokémon with large blue eyes like Mew and it has blue tipped antenna. It has a small pair of wings and light green arms and small darker green legs. There is almost always a smile on its face. Celebi probably originated from a nature sprite like in legends.

Pokémon 4Ever
In the Pokémon movie Pokémon 4Ever, it is the star/main Pokémon, along with another legendary Pokémon. It inadvertently takes Sam from the past while trying to flee a bounty hunter only to end up in the sights of a more sinister hunter.

Zoroark: Master of Illusions
Celebi also played a role in the 13th movie as Zorua's friend.

Pokémon Mystery Dungeon
A pink Celebi joins the players group in the world of the future, in the last dungeon before returning to the past. She has an unusual personality, and is an old friend of Grovyle, helping him go to the past twice. In the game, it is hinted that she may have feelings for Grovyle. Celebi will only join your team for a short period to reach a place in the main story, and can't be seen again unless you play the Special Episode "In The Future Of Darkness."

Super Smash Brothers
Celebi is also in Super Smash Bros. Melee and Super Smash Bros. Brawl. It is rare to unleash, and will only come out of a Poké Ball, like all Pokémon. Once Celebi is released, it drops down trophys while it flies upward toward the top of the screen until it disappears. You can also get the Celebi trophy by completing certain tasks in the game.

Trivia

 * Celebi was the second Pokémon introduced who is not legitimately obtainable in any main series game as of now, the first being Mew.
 * It is possible to obtain Celebi in the Japanese version of Crystal, by placing the GS Ball on the Shrine in Ilex Forest.
 * Celebi is the only Pokémon thus far that is able to time travel, other than Dialga, the Time Deity.
 * In HeartGold and SoulSilver, when you approach the shrine in Ilex Forest with a promotional Celebi, you will go back in time 3 years to battle Giovanni.
 * The only way to obtain a Shiny Celebi outside of Japan is through hacking or the Gamestop/GAME Event.
 * A hacked Celebi will not trigger the Zorua event in Black and White. Only Celebi from the GameStop/GAME event will have the bond with the Zorua in Castelia City. This also resembles the part in the thirteenth movie, where Celebi shared great bond with Zorua.
 * In PokéPark Wii: Pikachu's Adventure after you beat the game in the Meeting Place, there's a sign with a code on it that when you enter it, you can encounter Celebi.