Sonic the Hedgehog 4: Episode Metal
Have a saved game file from Sonic The Hedgehog 4: Episode 1 and complete Sylvania Castle Zone Act 1. If you then go to the 'Map' screen you will be able to play as Metal Sonic in Episode Metal.
Unlock Super Sonic as a Playable Character:
When you complete each of the special stages and obtain ALL 7 Chaos Emeralds Super Sonic will become available as a playable character. Note: You must collect 50 rings in a given stage.