Earn the following number of trophies: thirteen in Beginner mode; or seven in Standard mode; or five in Proud mode; or simply complete Critical mode. Then, correctly answer the three questions given to Riku at the end. The correct answers are:

1. Losing something that's important (third option).
2. My friends (second option).
3. To find something important (second option).

Then, unlock the Secret Message during the credits. Do this by collecting the gold letters during Sora's credit dive until the message "Secret Message Unlocked" is created. Save the game, then reload it. A message will appear, stating that you unlocked the secret ending in Theater mode.