Weighted Companion Cube

The Weighted Companion Cube, often referred to as the Companion Cube. is a recurring object in the Portal universe. It appears almost identical to the Weighted Storage Cube but with a pink heart replacing the Aperture Science logo.

In the first Portal, the Companion Cube was given to Chell to aid her in completing Test Chamber 17. In order to progress through the exit at the end of the chamber, the cube was dropped into the incinerator to be "euthanized". GLaDOS used this moment to taunt Chell during their confrontation.

In Portal 2, briefly torments Chell by repeatedly destroying Companion Cubes in Test Chamber  Another fact about the Companion Cube in Portal 2 is that it actually hums a song of some kind.

The Companion Cube makes a prominent appearance in the Rattman Comic, where it speaks as part of a reference to the first Portal game where GLaDOS states that the Companion Cube cannot talk back and should it ever speak, the test subjects are told to disregard its advice.

Trivia
Players who pre-ordered Portal 2 received a Team Fortress 2 Pin depicting the Companion Cube in a portal and players who played every game in the Potato Sack were also given another pin depicting the Companion Cube; both Pins share the same model.