Deoxys is an alien-like, bipedal Pokémon that has four forms, each focused on a different stat. The main components that are constant for all four forms are dominantly a red color, a light blue face, and three light blue dots on its back arranged in a triangle. Typically, there is a purple stripe running down the center of its face and a purple, crystalline organ in its chest. Its white, circular eyes are set inside rectangular, black eye sockets. It is shown in the anime that some Deoxys can have a green crystalline organ and facial stripe.

Deoxys was formed by the mutation of a virus from space. The Pokémon is highly intelligent; its brain appears to be the crystal in its chest. This organ is capable of firing laser beams. As shown in the anime, it is able to regenerate any part of its body that it may have lost during a battle. It has also demonstrated the ability to create an aurora without changing its form. These auroras can have different electromagnetic wavelengths and are used to communicate with other Deoxys. It is also shown that the various electromagnetic waves given off by people, Pokémon, and all electronics can obscure its vision if there is enough interference with its communication. This Pokémon came from outer space from inside of a meteor. Deoxys's different forms each possess different abilities and techniques.

In the anime and manga, Deoxys has been observed to create clones of itself. These clones have lifeless eyes and duller color, lack of the trademark crystal on the chest, and seem to be unable to change from the Normal Forme. Furthermore, they seem unable to morph their tentacles into hands. When these clones are defeated, they dissolve into dust-like particles.

Forms
Deoxys has four forms: Normal Forme, Attack Forme, Defense Forme, and Speed Forme.

Normal Forme
In its Normal Forme, Deoxys has tapering legs with a segmented, vertical, blue stripe on the upper portion. The abdomen is black with vertical lines, while the majority of its skin is reddish-orange. The arms are capable of taking different forms, usually a pair of tentacles coiled in a double helix. One tentacle is the primary skin color, while the other is light blue. Alternately, it can form more humanoid arms with a red top and blue underside and a five-fingered hand. This Forme has a blunt protrusion with a single blue stripe on either side of its head and stubby tail.

Attack Forme
While in its Attack Forme, Deoxys's legs remain tapered. However, its blue stripes are no longer segmented and run the entire length of the leg and there is now a large spike on the knee. Two pairs of whip-like tentacles with pointed tips, one reddish-orange and one blue, replace its previous coiled, blunted tentacles. The protrusions on its head are now triangular with an additional one extending from the top of its head. Most of its reddish-orange skin has receded, revealing a striated, black chest and abdomen. Finally, its tail is now longer and more pointed.

Defense Forme
Deoxys's Defense Forme has bulky, wide legs that are somewhat flat at the end with a bluish-green spot on each knee. It retains its two pairs of red and blue tentacles, but they are now flat and broad. The head has fused with its body and all protrusions are gone, causing it to resemble a helmet. The reddish-orange skin now covers the entire body, so none of its inner black portions are visible. There is now a single blue stripe on each shoulder, and its tail has disappeared.

Speed Forme
The Speed Forme of this Pokémon has a sleek black body with thin striation. The reddish-orange skin now covers only its head, hip joints, and the area around its crystal. There is now only one tentacle on each side of its body, one red and one blue. The protrusions on its head now point upward and the back of its head now forms a long spike. There is a long, thick blue stripe running down the center of this longer protrusion. More blue stripes appear on either hip joint and on its legs.

(Source: Bulbapedia)