Co-op mode is a mode where the player can get another player to help him or her with 1-Player modes over Wifi or directly.
Over an Internet Connection
Over the Internet, a player can get others to help with the Home-Run Contest game mode and the Multi-Man Melee game mode.
At Home
You can get a friend to help you with the Subspace Emissary, All-Star Mode, Boss Battles, Target Test, Home-Run Contest, or Multi-Man Brawl.
Subspace Emissary
The character allowance stays the same, but first player gets three characters (5 in the Great Maze) and the other gets one, rather than 1 player getting all of them. This can make things easier or harder depending on the stage the players are on. If they are on a stage that has lots of enemies, this can help because there are twice the amount of people helping to defeat the enemies and one more person that they have to gang up on. But on other stages, such as stages with a boss, this can be a burden. For example, if Tabuu uses his Off Waves attack, both the players may be KO'd, thus losing the players 2 characters rather than just one. Note that player 2 can join at anytime by pressing the start or + button.
All Star Mode
If two players attempt to do All Star Mode, they will go though the normal format with a few minor changes such as:
- The final foe-Olimar now has a second Olimar to help him out.
- If one of the players is KO'd, the other player cannot continue on his own, it is as if both players have been defeated.
- Instead of having 3 heart containers, the players now have five, the two extras being below the platforms that hold the normal three.
- At the end of the event, you get a combine two-player score instead of the two scores being recorded onto both the characters, although you receive both the trophies.
Boss Battles
Co-op Boss Battles has a very similar format to 1 player Boss Battles, with the few slight differences being very similar to All-Star mode:
- If one player is KO'd, the other may not carry on by his or herself.
- There are five Heart Containers.
- There is only one final score rather than two to be recorded onto the character.
Target Test, Multi Man Brawl and Home-Run Contest
Target Smash!!, Multi Man Brawl and Home-Run Contest are pretty much the same, only there are two people playing rather than one. Challenges for these can be broken in two-player; in fact, it is probably easier.