In order to unlock these characters in the game, you must do the following to access them:
Dr. Mario - To unlock Dr. Mario, you must complete 1-P Classic Mode using Mario at any difficulty without stopping.
Falco - To unlock Falco, you must complete the 100-Man Melee mode, and beat Falco after you complete that.
Ganondorf - To unlock the evil Ganondorf, complete Event 29. You will then be challenged by Ganondorf, if you defeat him, you’ve got it.
Jigglypuff - To unlock Jigglypuff, simply complete the game in 1-P Mode at any difficulty.
Luigi - To unlock Luigi in Adventure Mode, complete the stage with a 2 in the seconds meter (03:12:43 for example). You’ll then view a scene where Luigi comes and kicks Mario away so he can fight you with Peach. Defeat both, and you’re in.
Marth - To unlock Marth from the Fire Emblem series, play Classic mode with all 14 basic characters. Marth will then challenge you, and beat him.
Marth Method 2 - You can also use every character as a human player in VS. Mode, and the get a challenge from Marth.
Mewtwo - To unlock Mewtwo, play in VS. Mode for either 20 hours or 700 matches in that mode. After it’s met, Mewtwo will challenge you.
Mr. Game & Watch - To unlock Mr. Game & Watch, you must complete both Classic or Adventure 1-P mode or the Target Test with all characters at any difficulty, regular and hidden. After you do that, you’ll be challenged.
Pichu - To unlock Pichu, complete Event 37. Pichu will challenge you.
Roy - To unlock Roy, also from the Fire Emblem series, complete Classic Mode using Marth at any difficulty. Roy will challenge you.
Young Link - To unlock Young Link, complete Classic mode with 10 different characters at any difficulty. You’ll then be challenged by Young Link.
