Super Sonic – You must collect the seven Chaos Emeralds from the special stages.
True Ending – If you collect the seven Chaos Emeralds to unlock Super Sonic, you can then defeat the final boss to unlock the true final boss fight, followed by the true ending.
Knuckles Ending – From the secrets menu, you can activate “&Knuckles” mode. This gives you Knuckles as a companion for the game. If you play the game as Knuckles in “&Knuckles” mode, you’ll get a short special ending featuring Knuckles.
The Knuckles ending can also be unlocked if you use the debug menu/level select to fight the true final boss as Knuckles.
Earning medals from the Blue Sphere stages will unlock the following rewards:
Super Peel Out move - 1 medal
Insta-Shield - 6 medals
&Knuckles Mode - 11 medals
Debug Mode - 16 medals
Mean Bean Mode - 21 medals
D.A. Garden (sound test) - 26 medals
Blue Sphere Mode (new stages) – 32 medals