Celebi

Celebi or Serebii, the Time Travel Pokémon, is one of the Legendary Pokémon first introduced in the Pokémon Silver and Gold versions. Celebi was also the second Pokémon introduced who is not legaly obtainable in the game, the first being Mew.

Celebi is numbered 251 in the Johto-Dex.

Pokémon Mystery Dungeon
A crystal 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.