To unlock Blaze as a playable character, beat the boss of World 3 (Coral Cave).

Sonic Rush Adventure Unlockable: Music & Dialogue

You can find a koala playing a guitar on Windmill Island after beating the boss in Big Swell. By speaking with him, you can listen to all of the in-game music and dialogue, as well as more missions that will unlock music that would otherwise be unattainable.