Speed Socks
Finish the game on Normal. This prevents an S or A rank.

Extras:
Concept Art Gallery - Finish 10 Assignments
Sound and Music - Finish 20 Assignments with an S Rank
Ancient Hissatsu Techniques - Finish 30 Assignments
Memories (Video Gallery) - Finish 40 Assignments
Colored Rice - Finish 50 Assignments with an S Rank
Izayoi (Rikimaru's Unbreakable Sword) - Gather all of the Map Pieces in stages 1-5 and 10.
Onibi (Ayame's Unbreakable Sword) - Gather all of the Map Pieces in stages 6-9
Devil's Mask - Get all 20 Mask Pieces in Shadow Mode
Ninja Tabi '08 - Finish Story Mode