As a bird based Pokémon,it has huge flaps that are red colored internally and black colored externally .When Yveltal straightens its arms in flight, it forms the shape of a Y. This, along with the fact that Yveltal travels up and down, like the y axis, is why the game is called Pokémon Y.
Yveltal represents destruction, so it seems to be an angry Pokémon.
Yveltal is set for an anime appearance in M17 along with Xerneas and Diancie
Yveltal battled Xerneas and destroyed X and Y's hometown, Vaniville Town.
Its form may be a reference to Norse mythology guardians of the Yggdrasil tree: a bird-like creature that protects the top of the tree, this may be Yveltal, a deer-like creature that protects the soil and trunk of the tree, this may be Xerneas, and a snake-like creature that protects the underworld below the tree is Zygarde.
Yveltal starts with the letter Y because it represents the Y axis in the 3D axis of X, Y and Z.