He may seem like just a hand, but there's more to him than meets the eye. Being the big boss of the Super Smash Bros. series, Master Hand can be viewed as the orchestrator of all the Nintendo characters' fights. Did you ever make your action figures battle your hands as a kid? From the perspective of a toy, he's a tyrant of mayhem that brings endless fighting and unrest. His left-side counterpart is even crazier, too.
In fact, due to lazy sprite flipping, you may find yourself fighting the left hand instead during parts of his boss fight.