Giratina

Giratina is a dual-type Ghost/Dragon legendary Pokémon. While it is not known to evolve to or from any other Pokémon, it has a second forme activated by giving Giratina a Griseous Orb to hold. Its original forme, Altered Forme, will then become Origin Forme. it acts as an Antagonist in Giratina and the Sky Warrior.

Legend of Giratina
It was banished for its violence. It silently gazed upon the old world from the Distortion World.

Giratina and the Sky Warrior
Giratina is the only Pokémon that lives in the Reverse World. Its habitat absorbed all the pollution from the battle between Dialga and Palkia in The Rise of Darkrai, which causes Giratina to see Dialga and Palkia as its enemies. In a lake in the real world, Giratina lures Dialga to the Reverse World, hoping for revenge. However, Dialga escapes, thanks to the hole created by Shaymin's Seed Flare, and it traps Giratina in a time loop, causing the Renegade Pokémon to lose the ability to move to the real world. It then wanted to catch Shaymin to utilize its Seed Flare to create a hole to the real world, thus shattering Dialga's time loop.

Later, Zero catches Giratina after it had regained the ability to move to the real world, planning to copy Giratina's abilities and rule the Reverse World. Zero succeeds in getting Giratina's power of going from dimension to dimension, and as a result of Zero's actions, Giratina almost dies, but is healed by Shaymin's Aromatherapy. Giratina, along with Shaymin and Ash, defeat Zero and save the Reverse World. Afterwards, Giratina later goes back to the Reverse World, presumably looking for Dialga again.

Arceus and the Jewel of Life
Dialga saves Pikachu and Piplup from a spatial rift that appeared in a river. Giratina, still enraged over the damage done to its world, immediately arrives in Michina Town and attacked Dialga. Sheena was unable to touch Giratina's heart due to its anger. Ash then pleaded to Giratina to stop, and the Renegade Pokémon, after recognizing Ash as the person who had helped saved its life, stopped the attack and returned to its own dimension. Later, Giratina joined forces with Dialga and Palkia and helped to protect everyone when Arceus arrived to destroy Michina. Although it was injured, Giratina was healed by Arceus after the timeline was changed and returned to the Reverse World where it could live in peace once more.

Trivia

 * Giratina's Altered Form has six legs, six ribs, and six spikes on its wings which ties to both 666 the number of the beast and its similarity to Satan.
 * Giratina's role is similar to Satan on a cosmic level: it's a literal god that was banished for its aggressive tendencies(in particular, its obsession with balance) by what is implied to be the aforementioned Arceus.