I'm gonna say Sonic. Big surprise.
Tails, Knuckles, Eggman, even Shadow or Amy would fit in quite well really.
No.
Shadow represents the worst a company can do; fan pandering, wussing out of their own narrative for the sake of fanservice, lazy character designing, trying to appeal to a target demographic that isn't actually their main audience and thus alienating their original fans, demoting characters that were previously important to second fiddles, and trying to become 'darker and grittier' to show how mature they are.
I'll take Big the Cat over that piece of ****.
Although the other suggestions are acceptable. Even Amy. If we HAD to have a secondary Sonic rep I mean. I'd personally pick Tails, but Knuckles and Eggman are also solid choices.
Shadow the Hedgehog is literally the worst character creation this side of Baby Daisy.