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.
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You don't unlock him! If you unlock Sonic and play as him, Super Sonic is his final smash.
You don't, however you can hack Sonic's texture to make him look like Silver.
Sonic can fight Kirby in Super Smash Bros Brawl.
beat sonic with silver 10 times on battle mode on the stage outer space then you will see the challenger aproching screen and beat her and then youve unlocked her and thats it
In the cutscene before the final battle with Tabuu, Sonic will appear and shatter Tabuu's wings. Sonic can then be chosen as one of the characters to fight Tabuu, and then you've unlocked Sonic the Hedgehog!