"The Enrichment Center reminds you that the Weighted Companion Cube cannot speak. In the event that the Weighted Companion Cube does speak, the Enrichment Center urges you to disregard its advice." — GLaDOS
In 2007, Valve, creators of the hugely successful Half-Life line of games, released Portal — a first person shooter based not on combat, but on solving various puzzles in order to proceed. The protagonist, Chell, is a test subject at a laboratory called the Enrichment Center, which is run by GLaDOS, an eccentric supercomputer AI. Throughout the game, GLaDOS overwatches and talks to Chell as she proceeds through several very potentially lethal test chambers, using a portal gun to create portals that link one part of the test chamber to another.
Many of the puzzles in the game involve cubes, typically used to activate floor buttons that open doors. However, the Weighted Companion Cube, decorated with pink hearts, is introduced in one of the later puzzles, where GLaDOS informs Chell that the cube will accompany her throughout the test chamber and asks her to take care of it. GLaDOS then reveals that previous test subjects have started perceiving the inanimate cube as alive, and reminds Chell that "the Weighted Companion Cube will never threaten to stab [her] and, in fact, cannot speak." The Companion Cube is used for more than just pushing buttons — among other things, as a stepping stone in an otherwise impossible to climb staircase and as a shield against lethal, flying energy balls. As GLaDOS suggests, it is Chell's only friend, or at least the closest to one she gets at this point in the game. At the end of the level, one that requires that same cube to be used to solve most parts of it, Chell is forced to drop the cube into a fiery pit for "euthanizing" in order to continue to the next test chamber, an act which GLaDOS suggests has been a great moral difficulty to previous test subjects.
As Portal's popularity grew, the Weighted Companion Cube eventually became its mascot of sorts, leading Valve to create merchandise such as fuzzy dice and plush toys based on it.