Instructions
Play through every cup and get first place in every race while in 150-cc mode. This is the fastest mode of racing. Doing this unlocks Mirror Mode, which reverses all of the tracks.
Play through all of the cups in Mirror Mode until you can race in the Star Cup. This is the third racing tournament in the game.
Race in the Star Cup while in Mirror Mode and win the cup. You do not need to get first place in every race -- you just need to have the most points out of all the other drivers. When you are awarded the gold trophy, you will see a notification stating that you unlocked the Dolphin Dasher.