Die ten times.
Miss the same QTE action ten times.
Collect all nine Moji. Then, go to Chapter 5, and enter the new cave to get the large sword and upgrade it.
Successfully complete Chapter 8 to unlock the Ninja Blade.
Successfully complete all missions on the Hard difficulty to unlock the Ninjitsu Protective Barrier.
Get an "A" rank in all chapters, then upgrade the dual swords to the maximum to unlock the Tsuinetanarunaifu (Twin Eternal Knives) weapon.
Choose a very light color for your appearance in order to distinguish your character better from his background.