Unlock the following bonuses by performing the corresponding tasks:

BONUS - HOW TO UNLOCK
Amy - Talk to her in the Casino as Sonic after finishing Sky Chase.
Big - Defeat Chaos 6 as Sonic.
E-102 Gamma - Defeat Gamma after finishing Sky Deck as Sonic or Tails. Alternately, finish Twinkle Park as Amy and watch the FMAs after it.
Knuckles - Defeat him after finishing Ice cap as Sonic or Tails.
Miles Tails Prower - Finish the C mission of Emerald Coast action stage as Sonic.
Play Emerald Course in chao race - Beat the rest of the four courses.
Super Sonic - Succesfully complete the game as Sonic, Tails, Knuckles, Amy, Big and E-102 Gamma. Super Sonic will be avaliable only in Adventure Mode.