Alternate costumes
Successfully complete the game and save. Choose to start a new game and a "Shinnosuke Normal/Special" option will be available. The special version will put Samanosuke in a Panda costume with a baby panda in a front pocket clapping his hands. The Panda suit will be shown in most short clips but not all FMV sequences.

Onimusha 2 FMV sequence
Successfully complete the game and save. Choose to start a new game and a "Special Report" option will be available. This allows a small sequence from the game's sequel to be viewed.

Character change:
To get the character change you will need to pick up 10 pieces of fluorite.

Oni Spirits mini-game
Collect all 20 Flourites (blue rocks) in the game. Complete and save the game. A new speed trail mini-game will be unlocked.

Bonus stuff:
If you can beat the challenging Oni Spirits mini game, you will be well rewarded on your next game through. You will have infinite bullets for the matchlock, infinite arrows for the bow, and your magic will regenerate after you use the magic attack!

Bishamon O Flute
There are two places in there game where a very strange man hanging from the ceiling by his feet, stares at you and ignores you. Later in the game, after you use the Evil Plate and open the door, he will talk to you. The man will now take you (whenever desired), to the Dark Realm. In the Dark Realm, killing monsters until two portals open; one to go up and one to go down. Keep going down to the last level (20th) while collecting all the items in route. There will be no monsters to kill on the last level. Open the box to get the Bishamon O flute. Keep the flute just before the last fight with the Demon King.

Bishamon Sword
In the Demon Realm (just after you destroy the Ultimate Creation) is a save mirror and three doors. Go to the door that resembles an iron gate. It will not open at first. Use the Bishimon O flute on it and enter. Go to the Bishimon Sword jutting out of the Skull Demon wall object and take it. It is more powerful than all three of your weapons put together, and it has speed. The sword also has an unlimited supply of magic.