We've all tried to make an OC (Or I assume we have) Anywho, there are many people who just can't do it. First thing to do is NOT the appearance. I know, it's tempting as all get-out, but trust me. It's better to get the character's personality and such down first.
First, you should have a general idea of a character's past. If you get an idea like 'flying lizard monkey' (Not advised) Then jot it down. I hate when you have a great idea, and end up losing it while thinking too hard on how the character should say stuff or whatnot. If you jot down one sentence about them, then you can refer to that as you keep going.
Also... I know it's hard not to take lots of inspiration from the real sonic characters, but try to avoid making another off-colored Sonic with a bandana or something. Those make me (And others) puke blood. Instead, take inspiration from the personality. Is your character egotistical? Give him badass studded boots that can be used to impale his opponents. Is he nice? Then those boots should be avoided at all costs.
Also, and here's the most important part for me... NEVER make an OC too powerful. Seriously, reading about Chuck Norris the hedgehog would be like a straightening iron to the eyeballs, so just don't. Seriously, you don't need a bazooka to gain entrance to a door. You can just walk.
Give your characters some personality flaws too. Otherwise you end up with a Mary-su or Gary-stu. And nobody wants that. My character, Cane, for instance used to be a part of an army in the Northern Tundra that waged unjust war against the walrus populations (Chortle) When he left the Sosondowa (Inuit for wolf, I believe) clan, he escaped to New Mobotropolis, where he spent years trying to supress his temper.
See? Not only can a character flaw be interesting, but it can end up giving the character a drive, or reason to actually do what they do.
So on that note, make sure you give your character a drive. By that I mean, what are they after?
Shadow wants info on his past
Amy wants Sonic
Sonic wants both justice, and to escape Amy.
Vector wants to pay rent.
Silver wants to save his time? See? Without those things to guide them, how would their writers decide which decisions they'd make?
So in recap; Appearance only after personality. No Norris-spawns, and tons of personality issues.

So seeya later, hope I helped