Unlocking Super Sonic is the ultimate reward for completing the main game and collecting all the Chaos Emeralds. To do this, you must first complete the game with Sonic, Cream, Tails, and Knuckles. After completing the game with all four of these characters, you will unlock Amy Rose as a playable character. Once you have unlocked Amy and completed the game with her as well, you will then be able to access the final boss battle against Dr. Eggman and the True Final Boss. Upon defeating the True Final Boss, if you have collected all seven Chaos Emeralds across all your playthroughs, Super Sonic will be unlocked.
Super Sonic is not a playable character in the traditional sense; rather, he is a transformation for Sonic that can be activated during gameplay. Once unlocked, when playing as Sonic, if you collect 50 rings and then perform a double jump (press A twice), Sonic will transform into Super Sonic. In this form, Sonic is invincible (except to bottomless pits and crushing hazards), significantly faster, and his attacks are more powerful. However, Super Sonic's ring count constantly depletes, and if it reaches zero, he reverts to normal Sonic. Use Super Sonic strategically for speedrunning, easily defeating bosses, or getting through difficult sections.