ATLAS and P-body, referred to as Blue and Orange by GLaDOS, are a pair of bipedal Personality Construct based androids that are playable characters in Portal 2’s cooperative campaign.
Designed by GLaDOS sometime during the single-player campaign, they are to complete the Cooperative Testing Initiative in the Aperture Science Enrichment Center, previously uncompleted by human Test Subjects.
Gameplay-wise, ATLAS is played by the host of the co-op session (the player who sends an invitation), while P-Body is played by the guest (the player who accepts an invitation). PlayStation 3 players are automatically assigned as the host during cross-platform games, to prevent use of the console by the PC player.
The robots were designed from scraps. ATLAS' design is based on a Personality Core, while P-body is based on a Sentry Turret. Both had cores that were originally used in scientific calculators. To facilitate their humanization, they were given their own personalities, and clearly anthropomorphic designs and behaviors. Gender-wise, the robots have their own gender, as stated by Chet Faliszek in an interview (ATLAS as male, P-body as female).
Each robot has its own Aperture Science Handheld Portal Device that bears the color of its user (blue for ATLAS, orange for P-body), as two lines running along the barrel. Each robot's portal gun produces two distinct colors of portal (ATLAS' are blue (primary) and violet (secondary); P-body's are yellow (primary) and red (secondary)). Both pairs of portals can exist at once, and either can pass through each others' portals, a vital means of completing the tests. They are re-spawned through Apparatus Vents if destroyed. Duplicate robots are constantly built by automated machines, apparently keeping the memories and behavior of the previous incarnations.
Their heads function independently of their bodies, allowing them to control their limbs even if it is removed. This aspect of their design allows them to play pranks on each other by taking or knocking off their heads.