To unlock the true ending of Sonic Rush and gain access to the powerful transformations of Super Sonic and Burning Blaze, you must collect all seven Chaos Emeralds as Sonic and all seven Sol Emeralds as Blaze. This means successfully completing all 14 Special Stages. Once you have all the Emeralds for both characters, you will unlock the Final Zone.
In the Final Zone, you will play as Super Sonic and Burning Blaze in a combined, epic boss battle against the true antagonists. These transformations grant immense power, making you invincible and significantly increasing your speed and attack strength. However, they are temporary and consume rings over time. To maintain the transformation, you must continuously collect rings during the boss fight. Successfully defeating the final boss with both transformations will lead to the game's true ending. This is the ultimate goal for 100% completionists and a truly satisfying reward for your efforts.