The GS Ball used to capture Celebi
Celebi was the object of the Masked Man's longing, and, the only Pokémon he could never manage to capture on his own. His dedication to acquire it for his plans was so strong that he would visit the shrine in the Ilex Forest to attempt to catch it, not being aware of the existence of the GS Ball's purpose.
Celebi first appeared in The Last Battle IX, watching the Ilex Forest from its shrine. In The Last Battle XIV, the Masked Man completes the GS Ball using the Rainbow and Silver Wings and succeeds in opening the shrine and capturing Celebi. Pryce then used Celebi to take him back to a time where he lost something precious, his two Lapras. Gold dives in after them in an attempt to save Celebi, and eventually manages to after help from the other Pokédex holders. The GS Ball was destroyed by Pibu, and Pryce ended up stuck in the voids of time, where Celebi sends past memories of the song his old chums had written for him when his two Lapras died.
After the events with the Masked Man and before Creeping Past Cacnea, Celebi purposely joined Ruby's team before he moved to Hoenn, although he didn't even know what Pokémon it was. Celebi was kept in its Poké Ball for most of his journey, with Ruby only calling upon it as a last resort due to its powers being a mystery to him.
In Trying to Trounce Torchic, Ruby uses Celebi to defeat a wild Seviper after the rest of his and Sapphire's Pokémon are defeated, although its identity wasn't revealed in this chapter.
Celebi destroying the Red and Blue Orbs
In With a Spoink in Your Step I, Ruby and Sapphire travelled through the flow of time to reach Mirage Island. Since Ruby had the Time Travel Pokémon with him the flow of time was somewhat unstable. Upon leaving, Ruby and Sapphire experienced the same problems.
In It All Ends Now VIII, Celebi was used in the battle with Archie and Maxie by having it take the Red and Blue Orbs from them and breaking them into pieces. Using its time travel powers, Celebi brought the people who had lost their lives in the battle back to life. With its duty fulfilled, Celebi sent Ruby away to be with his friends and left.
Prior to All About Arceus VII, Celebi sensed the destruction that Arceus would bring upon the Johto and Sinnoh regions. It subsequently released Pryce from the Crack of Time that he entered in years ago so he could help in the battle. Celebi was later seen collecting herbs to cure Giovanni's illness, and was assisted by a pair of Pichu and the Team Rocket Elite Trio. It was soon attacked by several Team Rocket Grunts, but Ken, Al, and Harry managed to get the stolen herbs back from them, and urged Celebi to get to the Ruins of Alph immediately. Soon enough, they reached Giovanni, and Celebi gave its medicine to him, curing him from his long-time illness.
Celebi is a caring individual, even helping those that have done it harm. It has a strong sense of duty, and often goes across the world in order to help fix times of crisis.
Celebi is able to travel through time, with one way going through the Ilex Forest's shrine. It is able to fly using the pair of wings on its back. Celebi is also able to alter the timeline, having undone certain people's deaths during the crisis in Hoenn.
