To play through the enitre game as Momiji, select the hard difficulty level, survive the fight with Ryu and successfully defeat the dragon boss. A new difficulty will be unlocked - The Way of Kunoichi. Begin a new game to access it.
If you have set the DS to your birthday, you will get a special "Happy Birthday" sound effect when you start the game! Instead of the usual sound effect when you tap the start screen, you'll hear one of the characters yell out "Happy Birthday!"
Complete all chapters to unlock the "Prizes" option and Hard difficulty. The "Prizes" option allows you to view intermission sequences, letters, and character files that were found during your game session. Most are unlocked during the game by shooting the birds that appear when you use the microphone when a bird sound is heard.