What exactly is it anyway?
NEVER QUESTION THE COMPANION CUBE OR YOU SHALL BURN!
Since the big pile of "surf control" at work, i'm forced to only give you the "pre-link" to the pictures of the Companion Cube.
forums.gametrailers.com/showthread.php?t=199138
And its just basically kind of a "joke"...if you watch spec's video, listen to what the voice is saying about it..."cannot speak and will not stab you" and what not. They got lots of little funny things in the game such as the "cake" for your party lol. But anyway, to sum it all up, the companion cube helps you through obstacles you wouldn't be able to get past (using it as a ledge to climb high walls and what not)