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In the movie, Shaymin is known as the Gratitude Pokémon. It is able to communicate with other people through telepathy. After Shaymin was accidentally pulled into the Reverse World by Giratina and got caught in the middle of the battle between Dialga and Giratina, it used Seed Flare to create a portal out of the dimension. This strange experience caused Shaymin to become terrified of almost everything. It also constantly referred to Gracidea, and was convinced that Giratina wanted to eat it when the Renegade Pokémon really wanted to use Shaymin to open a portal to the real world.
Zero also wanted Shaymin as it was the only one who could make a portal back into the Reverse World so as to capture Giratina. But after being exposed to the Gracidea and transforming into its Sky Forme, it was able to stop Zero from fulfilling his goal.
Shaymin's personality changes depending on which forme it takes. In its Land Forme, it's very fearful, timid and rather pompous while it is brave, outgoing and friendly in its Sky Forme. It also uses different speaking dialects in each forme in the Japanese version. In the English dub, Land Forme has a feminine voice and Sky Forme has a slightly masculine voice. Shaymin would transform into its Sky Forme when it came into contact with Gracidea pollen. It would revert back to Land Forme at night or when it was exposed to ice.
Shaymin covered in flowers
Contrary to its cute appearance, Shaymin actually had an incredibly large ego and would boss people around. While Dawn liked Shaymin despite this, Ash commented that it was mean and it would explode randomly. Shaymin enjoyed being held by Dawn and also riding on Ash's head. It wouldn't move from there as it wanted Ash to also suffer if it was in trouble. Due to all this, it appeared that Shaymin and Ash disliked each other.
After Shaymin made it to Gracidea, it admitted it never truly disliked Ash. It was grateful to all Ash and his friends for all that they had done for it and after bidding a tearful farewell, it joined the other group of Shaymin at its home and they all turned into their Sky Formes and flew off.