Unlock all five normal endings. A special prologue will appear, allowing you to play again and unlock the "Extra" ending.
Extra option
When the game is completed, one of five endings will appear. An "Extra" option will now be available at the title screen, which allows you to view the percentage completed for every chapter. You can also start the completed game over again at the beginning to try to do better and get a different ending.
FMV sequences
Successfully complete the game to unlock a FMV sequence from the Spring 2000 Tokyo Games Show. Successfully complete the game after getting three different endings to unlock a FMV sequence from a private Konami game show in Europe. Successfully complete the game after getting all five normal endings and the "Extra" ending to unlock a FMV sequence from the Fall 2000 Tokyo Games Show.
Restore energy units
If you begin running out of energy units, allow yourself to die. The moment you are brought back to life, you will get an energy unit for free. For example, if you let yourself die five times in a row you will get five energy units for free. Do this trick each time you begin a new chapter.