Peer Review

Peer Review is a free DLC made for Portal 2 aimed at expanding upon the co-op part of the game.

It continues the story of ATLAS and P-body as they assist GLaDOS in finding a mysterious intruder in the facility through the Cooperative Testing Course "Art Therapy", and adds a challenge mode for both singleplayer and co-op maps, allowing a player to race certain maps, trying to place the least amount of portals in the shortest amount of time, as well as compare their scores with other users.

Characters

 * ATLAS and P-body
 * GLaDOS
 * The bird and her three chicks