Win Shadow Cup in Single Player mode.
you have to beat crisis city (or flame core) and you'll unlock shadow.
Some of the suits are only unlocked by playing multiplayer
You don't unlock him! If you unlock Sonic and play as him, Super Sonic is his final smash.
Simply beat the final stage, mission 3. THe rival duel between Sonic and M.Sonic. In order to get the final stage you must clear the all team stories. (ROSE, DARK, BABYLON, and HEROES)
in sonic adventure dx you can only unlock metal sonic by collecting 130 sonic emblems.
hes only a rival in sonic generations.
Sonic would be Mario's rival from Sega.
Super Sonic is Sonic's Final Smash; you cannot unlock him as a real charactor though.
Your question's a bit open-ended. Unlock Sonic for which game, exactly?
You can't get Shadow on Super Smash Bros Brawl, though you can change Sonic's colour to make him look like Shadow.
You can't (But to unlock Shadow + Amy, you type in 5953131313131313)
you can't