Do the following to get it

Unlockable:	How to unlock:
Dark Dragon Blade	turn in 50 Scarabs in hard mode
Plasma Sabre Mk.II	turn in 49 scarabs in hard mode
Clone Ryu outfit	beat normal or ninja dog mode
Red Muffler outfit	beat hard mode
Hotsuma Outfit	beat very hard mode
DOA Throwback outfit	beat special misson
Speical misson	complete the game
Ninja Gaiden Catridge	turn in 50 scarabs in ninja dog or normal mode
Armlet of the potency in hard mode	Beat Clone Ryu in the archives room
Ninja Dog Mode	Die 3 times in chapter 1 and answer yes for every question