The various characters within the game are unlocked by completeing various objectives. Mostly completing the tournaments, singles and doubles, in each level of difficulty will unlock the corresponding character.
Sonic Underground is an animated TV series. And no, you cannot unlock episodes, or characters other than Sonic, from Sonic Underground in Super Smash Bros Brawl.
Sorry, but there is no character as 'super sonic' in the game ultimate flash sonic, the only unlock-able characters are Amy Rose and Shadow The Hedgehog. For all the unlocked characters+all cheats+the full Juke box type in the password 595313131313131.
You have to get all the coins in each race/level, unlock all characters, then, you can use Super Sonic in Grand Prix. I beat the game.
You don't unlock him! If you unlock Sonic and play as him, Super Sonic is his final smash.
You have to get 100% on all the characters stories. (Note: Sonic has to go through ten levels while everyone else has to go through five)