2.) Your best friend.
2.) While it has been a faithful companion, your companion cube cannot accompany you through the rest of the test. If it could talk - and the Enrichment Center takes this opportunity to remind you that it cannot - it would tell you to go on without it because it would rather die in a fire than become a burden to you.
The Companion Cube is only used in one level of the game, but it leaves a lasting memory and somewhat of a motive for the main character to "kill" GLaDOS, the antagonist of the game.
The main character, however, does not seem to have as much of a link or heart felt connection as does most other test subjects of the Apeture Science Computer-Aided Enrichment Center. This is shown by graphetti drawings on the walls in a hidden room within the level of the Companion Cube. At the end of the level, the main character is forced to euthanize the Companion Cube in a pit of fiery liquid metal. After which GLaDOS congratulates the main character and comments that she is the quickest of all test subjects to euthanize the Companion Cube.
At the end of Portal, the main character finally receives the cake in a brief party in which the Companion Cube is invited and surprisingly shows up.
The Companion Cube is your very best friend in the world, is male, cannot talk (though in the event that it does, Apeture Science strongly disregards listening to any of his suggestions), a...
It is a cube with hearts on the sides given to you in the valve game portal. You use the cube in only one level and it is shortly thereafter incinerated, yet its small appearance has gathered a cult like basis.
The cake is a lie, unlike the Weighted Companion Cube.