Celebi's wooden construct, in the image of a dark, mutated Celebi
While trying to escape from a Pokémon Hunter, Celebi stumbled across Sam, who was out looking for Pokémon. Just as the hunter was about to capture the Pokémon, it time traveled with Samuel Oak to escape and ended up in the time of Ash and his friends.
However, there was still danger in the present day, this time from a Team Rocket member known as the Iron-Masked Marauder. Using a Dark Ball, the Iron-Masked Marauder captured Celebi to use for his own evil plans, and the Pokémon was put under his dark control. With the help of Ash and the others, Celebi was set free from the Iron-Masked Marauder. However, it looked very grim for Celebi, who wasn't able to heal itself in the Lake of Life where it had healed before because the lake was too polluted.
Fortunately, many other Celebi from the past and future came to heal their friend. Once Celebi was healed, it was able to go back to the past and take Sammy with it.
Celebi is a very playful Pokémon, as it is often seen playing around with Ash and his friends. However, this playful and kind personality is masked after Celebi is captured in a Dark Ball by the Iron-Masked Marauder and put under his control. Instead of being its usual playful self, Celebi becomes a dark, hateful Pokémon. This behavior results in the creation of a giant Celebi out of surrounding branches and a massive attack on the forest it once called home. This behavior remained until Ash, Sammy, and Suicune managed to restore Celebi's memories and its old personality. After it is freed from the Iron Masked Marauder, Celebi retains none of its previous nasty and dark behavior. Celebi is also very helpful and spirited, however, due to this, it was an easy subject to the Iron-Masked Marauder's plans.