While it is not known to evolve into or from any other Pokémon, Xerneas has two forms, which change depending on its status. When Xerneas is in a battle or a Pokémon Contest, it is in Active Mode and its horns are multicolor; otherwise, it is in Neutral Mode and its horns are cyan.
Xerneas is a quadrupedal, stag-like Pokémon with four pairs of antlers. Its coloration varies slightly depending on whether it is in Active Mode or Neutral Mode. Regardless of mode, its main body is always black with a blue stripe down its back and a short, bushy tail. Starting at its knees there is a blade-like trim around each leg, which forms a point under each hoof. There is an upside-down, V-shaped marking on its chest in the same color as the trim on its legs. Its head and neck are blue, and there are two large, tufts that extend onto its chest and curve upward. Its first pair of antlers extends onto its head and curves around the underside of its eyes. The eyes are black with X-shaped pupils. There is a short marking on top of its snout and an upward curving horn on each side of its head in place of ears.
In Active Mode, its markings, antlers, and trim on its legs are pale gold. Its antlers have branches and bumps in many colors and there are multicolored spots on its back. Xerneas assumes this Mode during battle or when using its power. Xerneas is at full power when in Active Mode.
In Neutral Mode, its markings and antlers are instead pale blue and the embellishments on its antlers and spots on its back are a darker blue. Xerneas assumes Netural Mode when not battling and traveling. This remains until it needs to use Active Mode.
Xerneas has the power to share eternal life. This occurs when the horns on its head shine in seven colors. Xerneas can use its life energy to create vast forests and restore life to them. It can infuse both humans and Pokémon with energy and vitality. Because of this, Xerneas is highly respected by the ancient people of Kalos for its role in giving life. Xerneas is also known to be able to speak through telepathy. According to legend, Xerneas is known to have rescued wounded Pokémon during an ancient war. Xerneas and Yveltal have the power to transform Evolution stones into Mega Stones. Both are in charge of taking care of the land in a delicate balance between life and death. Xerneas is capable of calming Yveltal should it get enraged, as seen in the anime. Xerneas's power can also be used to power up machines such as the ultimate weapon.
A special Xerneas is worshipped in Lental for being one of the seven guardian Illumina Pokémon, in particular for saving the region from a meteorite 2000 years ago. It is also believed to be the originator of the Illumina phenomenon. It resides in the Aurus Island Illumina Spot.
A Xerneas appeared in The Legend of X, Y, and Z! during a flashback, where Jan asked for its help after Yveltal caused trouble upon his kingdom, including turning Aila to stone. Though it restored much of the damage Yveltal inflicted, it could not revive Aila, and thus left.
Xerneas debuted in An X-cuse to Come Out and Play, alongside Yveltal. The two faced each other in a battle, which caused chaos and destruction in Vaniville Town. The two eventually stopped fighting and returned to their dormant tree and cocoon states, which were sought after by Team Flare to power the ultimate weapon. Later Xerneas awakened and took back most of the energy that had been absorbed from it. Afterwards, it was befriended and caught by Y, who nicknamed it Xerxer. When Malva arrived and revealed that she had captured Yveltal, the two nearly clashed again before Xerosic stopped them, stating that Lysandre ordered a ceasefire, as the powers of the two would cause an endless war. Xerxer agreed with this and escaped carrying Y and her friends. During the final battle against Team Flare, Xerxer and Yveltal clashed again, and eventually chose to return to their dormant states in order to prevent Lysandre from using Zygarde to his own deeds.
Xerneas appears as a Poké Ball Pokémon. In the 3DS version, it is unlocked after playing Target Blast at least once, while in the Wii U version, it is unlocked by breaking at least 200 blocks in a solo game of Trophy Rush as Pikachu. When released, it uses Geomancy to energize nearby fighters, giving their attacks stronger knockback for a time or giving them stronger defenses.
Xerneas also appears as a trophy in both versions of the game.
NA:The horns on Xerneas's head give this Fairy-type Pokémon the power to bestow eternal life. It emanates energy with a move called Geomancy that causes all the fighters to glow. As pretty as it is, this makes everyone more easily launchable. Know when to take advantage and when to lay low.
PAL: The horns on Xerneas's head give this Pokémon the power to bestow eternal life. When summoned in a match, energy will emanate from it and make all fighters glow. Pretty it may be, but this Geomancy move makes everyone much easier to launch. Knowing that, will you take advantage of the situation, or try to stay out of the way?
Xerneas's horns and some highlights in its fur are light blue in Neutral Mode and light gold in Active Mode. In Active Mode, the protrusions on its horns and the spots on its back light up in several different colors.
Xerneas has the same stat distribution as Yveltal.
Xerneas, along with Yveltal, was designed by Ken Sugimori and Yusuke Ohmura. The initial designs for the two Legendary Pokémon were started by Sugimori, but he encountered "artist's block". For the first time, he handed the work over to another designer, Ohmura, who furthered the designs, after which Sugimori returned to finish them.
When a Shiny Xerneas is obtained by event, its model on the Mystery Gift screen is incomplete. It appears in its Active Mode, except the multicolored additions to its horns are either missing or the same blue color as on its neck.
Xerneas was designed so that its silhouette resembles the letter X, and by extension possibly the saltire symbol in heraldry or the X mark. Appearance-wise, Xerneas resembles a cervid, more specifically the extinct species Eucladoceros.