The co-op mode allows you to play through the story part of the game with a friend.The online co-op is more challenge based.
More info can be found here
Super Smash Bros. Brawl is the next installment in the Smash Bros. series for the company's Wii console. Among the new characters playable in the game are Meta Knight, the sword-wielding nemesis of Kirby; Pit, the angelic archer from Kid Icarus; Zero Suit Samus, the powerful Metroid series heroine minus her versatile armor; and Wario, who demonstrates a noxious attack of gastronomic proportions. Also Snake, the gritty soldier from Konami's hugely popular Metal Gear series. [Nintendo]
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