To unlock the characters:
Clear classic mode as Ness, or complete story mode with any character.
Play on the Pokemon stage as Pokemon Trainer, or beat story mode with Ivysaur.
Beat story mode as Luigi, or with any character.
Clear classic with Princess Peach and Princess Rosalina, or complete story mode with any character.
Obtain 5078 trophies, or clear story mode with any character.
Play on the Green Hill Zone stage as Sonic or complete story mode with any character.
Play on the Castle Siege stage as Ike or complete story mode with Roy.
Beat classic as Link and Toon Link or play story mode with Link.