Characters:
Ninja: Complete one of the Star Roads in the Star Cup (once you unlock him once you wll have him unlocked in all other sports).
White Mage: Complete both Star Roads in the Star Cup.
Black Mage: Complete the Star Cup 4 times.
Slime: Play 28 games or defeat him in a alternate route in the Flower Cup.
Moogle: Complete the Mushroom Cup.
Catuar: Play 60 games.

OR

If you wanna get EVERYTHING (characters, colors, and stages) you can highlight the file you want everything on (don't press A) then hold C and press 111 then hold Z and press 1111 then hold C&Z and press 11111! There you go you have EVERYTHING! Thanks to oreosrdelicious for the code!

Music: Waluigi Pinball - Mario Sports Mix