Cheat: Street Fighter Costumes
The last mainline Onimusha game had quite a few unlockables.
The most well known is the one where you get to play dress up with Street Fighter costumes. To get these outfits, do the following: at the menu screen, select Special and put the cursor on 'Exit'.
Then enter these codes: Cammy costume for Jubei: L2, L2, Right, Right, L3, Tringle, Left, L1, L2, Square Chun-Li costume for Ohatsu: R3, Right, L2, Left, Left, R3, L1, R1, Right, R3 Guile's costume for Roberto: R2, L2, Left, L1, Left, Right, R3, Square, Square, Triangle Ryu's costume for Soki: Left, Triangle, R2, R3, Square, R1, R1, Right, Left, L2 Ken's costume for Tenkai: L3, L3, R3, R3, R3, Left, R2, L1, Square, Right.
Involved? Yes. Worth it? That's your call.