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Crazy Hand is the secondary antagonist of the Super Smash Bros. series, and is portrayed in SMG4's videos as the left-handed brother of Master Hand.
Crazy Hand appears in SMG4: The New Smash Bros, taking over for Master Hand due to having a case of arthritis. Instead of doing the original game modes, Crazy Hand creates new "fun modes" for the roster to compete in. Which is a series of insane and completely unfair modes made specifically for thrashing the contestants for his own amusement. Master Hand returns to put a stop to Crazy Hand's deranged activities and start re-hosting the game modes.
While he has the same malicious intent as his brother, as his name states, he's crazy. In contrast with the trolling yet manipulative nature of his brother, Crazy Hand escalates his brother's aspects to different levels. This is shown that whereas Master Hand makes his games at least fair and sufficiently deadly, Crazy Hand outright makes his games unfair and brutal with no concern of how it goes, at one point considering bank robbery a fun game.
With outright sociopathy and clear sadism, he's basically his brother with his negative traits magnified and none of his self-control.