Master Hand is one of the more mysterious entities of the Super Smash Bros. franchise, appearing in each game at the end of their own single-player modes. Master Hand takes on the appearence of a floating, white glove, resembling that of Mario himself, using many hand-based attacks in an attempt to harm the player.
During the events of Melee, Master Hand was revealed to have a "twin", the left handed Crazy Hand, who features slightly more sporadic attacks as his name would imply.
In Super Smash Bros. 4, Master Hand turns into an odd entity known as Master Core when he gets enough damage and difficulty 5.1 and over. It starts out as a mass of darkness with multiple forms, until the last one, which is a Smash-Ball like orb.
Master Hand's mysterious nature has led many fans to try and figure out exactly what he represents; the strongest theory involves a cutscene from the first game implying that he is in fact the hands of someone who brings the characters to life.