Yveltal possesses avian and draconian traits. It has large wings and a tail that are red colored internally and black colored externally. It has intense teal-colored eyes with white pupils. When Yveltal straightens its arms in flight, it forms the shape of a Y. Its talons are also Y-shaped. Yveltal is part flying type, and based on the concept of the y-axis.
Yveltal represents destruction, such as natural disasters, while its counterpart, Xerneas, represents new life.
When Yveltal is about to reach the end of its lifespan, it steals the life force of everything living around it, then transforms into a cocoon to sleep.
Despite being a force of destruction Yveltal has been shown in the anime to not be inherently malicious: in "Diancie and the Cocoon of Destruction" after it was calmed by Xerneas it flew off with no further emnity to anyone.
It is likely that Yveltal, being a force of nature, is not outright evil but a morally-neutral force that causes devastation but is not ultimately doing so out of any wicked intent.
Its Shiny coloration resembles a decaying carcass, with the white parts resembling exposed, bleached bone with some fragments of decomposed flesh (in red) still attached.
Yveltal may be based on the eagle that guards Yggdrasil in Norse legend. It may also be based on a carrion bird such as a vulture, which represents death, or perhaps even a twisted inversion of a phoenix (with it symbolizing new life and rebirth while Yveltal represents oblivion and destruction).
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. It is part of Yggdrasil Trio.
Yveltal starts with the letter Y because it represents the Y axis (or the vertical one, and as Yveltal is in the sky, it’s a perfect fit!) in the 3D axis of X, Y and Z.