If you're a really successful game character and you have a lookalike, it's a good idea to give said lookalike his own game, however much you hate him. So, following in the footsteps of many game characters, Sonic the Hedgehog homing attacks out of the way for a bit. As you can surmise,
Shadow the Hedgehog diverts attention away from the happy-go-lucky blue 'hog, instead concentrating on his mysterious, dark, emotionally void lookalike (who, let's be honest, has dominated the series since
Adventure 2).
Let's recap (spoilerphobics might want to skip this paragraph). Shadow (supposedly) fell to his death at the climax of
Sonic Adventure 2. Later, he was discovered in stasis by fellow Emerald hunter (and borderline furry) Rouge, a happy reunion which was later shattered when she discovered a warehouse full of Shadow robots in similar stasis booths on Eggman's battleship. So Shadow, poor lad, has been left not knowing whether he's the real McCoy or...