Red Planet by Robert A. Heinlein

Willis was a bouncer, a Martian roundhead who looked for all the world like a hairy medicine ball and who could - and often did - function as a complex recorder of what went on around him. To Jim Marlowe, he was simply a friend; and wherever Jim and his pal Frank went, Willis went too. But they didn't go too far, since their forays were limited by strict rules. Then one day Willis unwittingly turned into a treacherous plot that threatened all the colonists on Mars...and that set Jim off on a terrifying adventure that could save - or destroy - them all!