Super Sonic is a form Sonic the Hedgehog takes, after obtaining all 7 Chaos Emeralds. This appears in SSBB with a currently unknown role, although it's generally believed that it will be his Final Smash. This theory is backed up by the fact that, in the trailer, right before Sonic transforms, he is on fire, much like other characters when they perform their Final Smashes.
When Sonic transforms, he gains the ability to float around the level at will. Any opponents he collides with will take high damage upon impact.