Unlockable: Secret Forms
Chaos Zero - Get every card in the game.
Metal Sonic (2nd form) - Complete Metal Sonic's storyline, and get the Metal Madness, Metal Overlord, and Neo Metal Sonic cards.
Super Shadow - Unlock all of Shadow's suits, complete his storyline, and beat all of the challenges as Shadow.
Super Silver - Unlock all of Silver's suits, complete his storyline, and beat all of the challenges as Silver.
Super Sonic - Unlock all of Sonic's suits, complete his storyline, and beat all of the challenges as Sonic.
Tikal - Unlock all of Knuckle's suits, complete his storyline, and beat all of the Colosseum Highway challenges as Knuckles.
Submitted by Manic the Hedgehog

On any stage with any character, if repeatedly press left and right (without moving, just changing direction) you will get smaller and float and then disappear, if your opponent goes by or if you move, you will go back to normal.

to get medal sonic beat all the story with
-sonic
-knuckles
-shadow
and
-silver

it's really long
god luck