The Weighted Companion Cube (also simply called the Companion Cube) is a fictional item featured in the Portal series of video games by Valve Corporation. Initially featured in a single level of the original Portal, Test Chamber 17, as one of Aperture Science's ubiquitous Weighted Storage Cubes with heart symbols printed on the outside, it is given to the game's main character, Chell, as part of the antagonist GLaDOS's sinister testing initiative. After carrying it through the entire level and ostensibly anthropomorphizing and "bonding" with the Cube, the malevolent AI forces her to unceremoniously dispose of it in an incinerator device. Companion Cubes later re-appear in the game's sequel with a slightly different design. The original Companion Cube is shown to have survived the events of both Portal and Portal 2, appearing as part of an ending gag.
While GLaDOS has suggested the Companion Cube may be sentient, it is unclear whether this was solely to psychologically torment Chell. However, in the comic Portal 2: Lab Rat, Doug Rattman's Companion Cube is shown speaking to him, possibly as part of a schizophrenic hallucination.
Following the game's release, the Weighted Companion Cube quickly increased in popularity among fans, spawning a wide array of official merchandise and fan works. It has since become a mascot for Valve's games, and has also been referenced in other, unrelated games as Easter eggs.
The Companion Cube was initially conceived as part of the gameplay of Test Chamber 17, due to players constantly forgetting to bring the crate along with them. Making the Weighted Companion Cube visually distinct and stating it as being so informed the player that it was of importance and was to be used and reused for the duration of the Test Chamber. The Cube was made to appear "vulnerable" rather than ubiquitous through GLaDOS's dialog, conditioning players to feel responsible for its well-being and want to take it along.
In early versions of the game, the player was forced to simply abandon it and move on.