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. The Masked Man eventually completes the GS Ball using the Rainbow and Silver Wings and succeeds in opening the shrine and capturing Celebi. The Masked Man 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. After being released, Celebi succeeds in healing the pain in the Masked Man's heart.
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, and was only used to end 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.
Celebi is shown to be 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 peoples deaths during the crisis in Hoenn.
For more information on this Pokémon's species, see Celebi.