Deoxys is a bipedal, humanoid, extraterrestrial Pokémon that has four forms, each focused on a different stat. On its chest, there is a purple, crystalline organ, which is said to be its brain; as seen in Destiny Deoxys (M07), this organ can appear in other colors, namely green. It has a light blue, oval pattern with a purple stripe running down the middle on its face; white, circular eyes set inside black, rectangular eye sockets; and three light blue dots arranged in a triangle on its back.

Deoxys is the result of a mutation of an extraterrestrial virus caused by exposure to a laser beam. It is highly intelligent and can fire off lasers from the crystalline organ on its chest. While inactive, Deoxys takes the form of a triangular stone tablet. Deoxys is said to appear where auroras are seen and causes one to appear when changing forms; in Destiny Deoxys, the aurora's electromagnetic wavelengths are used to communicate with other Deoxys, and other sources of electromagnetism were shown to obscure its vision if given off in great amounts. Moreover, in the same movie, it demonstrated the ability to regenerate any part of its body that it lost during battle and to create clones of itself, a trait also depicted in Pokémon Adventures. The clones are much duller in color, lack eyes or the organ on their chest, and seem to be unable to transform. Deoxys possesses a strong rivalry with , with the two often clashing with each other in space, as depicted in several media. During Pokémon Omega Ruby and Alpha Sapphire's Delta Episode, Deoxys was coming towards the planet inside the Grand Meteor Delta.

Forms
Deoxys has four forms: Normal Forme, Attack Forme, Defense Forme, and Speed Forme. As seen in various media, Deoxys can transform into any of its four forms willingly or when exposed to a Meteorite. Deoxys will change its forms depending on the situation, mainly staying in Normal Forme when not in danger.

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 morph its tentacles into 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. It is the most balanced form.

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. It is the most offensively powerful form and is said to possess an aggressive disposition.

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. It is the most defensively capable form.

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. It possesses the greatest agility out of the four forms.

(Source: Bulbapedia)