Xerneas is a legendary Pokemon in Kalos with the power to give life which is said to appear every 1000 years. On the day that Prof. Sycamore's package arrived in Vaniville Town, it appeared alongside its counterpart, Yveltal, and the two wrecked havoc as they clash against each other. Afterwards, it turned into its dormant Tree Forme. Later, it was discovered by Team Flare and brought back to its base to fuel the Ultimate Weapon. When it awoke from its dormant state, it decided to work with humans and chose Y as its trainer. It was once temporarily under the Pokeball Jack control of Essentia. At the end of its battle with Yveltal, it reverted back to its dormant Tree Forme.