Scourge was originally Anti Sonic / Evil Sonic. He came from an alternate universe, where things were far more chaotic and out of wack in comparison to the Mobius our heroes reside in. This Anti Sonic fought many battles against the real Sonic, before a fateful event occurred. He attempted to steal the Master Emeralds power, which ended up changing his body. Now Sporting Green fur, and blue eyes to contrast Sonic's Blue fur and Green eyes, he dubs his newfound self as Scourge, and continued to terrorize the heroes for many years to come. This psychotic hedgehog is not to be trifled with unless you have the backbone to stand up. 

(Scourge is an alternate costume for Sonic and Super Sonic in the Free Battle / Multiplayer modes rather than his own Character slot, and will therefore not have any story related lines.)

Scourge is Sonic, but the cockiness dial turned up to 11, and an overall meaner mug to deal with. This should carry as his Voice should sound like Sonic, but if Sonic was a smooth talking, evil person. You can take liberties with your interpretations, but I'd like to see him with something like a Boston or Ausie accent.