Get Bishamon Sword:
Kill all of the monsters on the 20th level of the Dark Realm and open the treasure box there to get the Bishamon Ocarina. Then in the area just beyond the second Marcellus boss fight use the Ocarina on the bone door here to open it. Inside is the Bishamon Sword which has unlimited magic which will kill any non-boss character in a single swipe. 

Get Ogre Armor:
Battle through the Dark Realm that has ten levels. Upon completion of the tenth level, open the chest to get the ''Secret Key''. Take the key to the area with the Red Minotaur boss fight. Use the key on the large door marked with a plus sign to enter an area that leads to the Ogre Tower. Once inside the Ogre Tower, enter the portal in the middle of the room. Defeat the enemies on each of the 6 floors, to unseal the next portal. When you return to the main level of the tower, give your souls to the glowing statue to unlock the corresponding portal. Repeat this process for the remaining statues. On the 5th and final portal, you must face a boss on the final floor. Defeat him to gain the ''Ogre Armor'' 

Unlock Easy Mode:
Die 5 times in a row in Normal Mode. 

Unlock Difficult Mode:
Beat the game in under three hours to unlock Difficult Mode 

Unlock Oni Spirits Mini Game:
Collect 20-29 pieces of Flourite 

Unlock Ultimate Mode:
Complete 12 stages of Oni Spirits Mini Game. Ultimate Mode lets you start Samanosuke with the Bishamon Sword, Infinite Normal and Fire Arrows and Infinite Bullets and Burst Bullets 

Unlock Kaede's Second Costume:
Collect all 30 Flourites 

Unlock Samanosuke's Second Costume:
Beat the game on Difficult 

Genma Onimusha lets you score your progress based on the point below. The perfect score is 30. 

Time taken to complete: 

Up to 3 Hours = 10 points
3-4 Hours = 7 points
4-5 Hours = 5 points
5+Hours = 3 points 

Number of demons killed: 

0-399 = 3 points
400-499 = 5 points
500-599 = 7 points
600+ = 10 points 

Number of souls collected by the gauntlet: 

0-34,999 = 3 points
35,000-44,999 = 5 points
45,000-54,999 = 7 points
55,000+ = 10 points 

The points add up as follows: 

0-9 = D
10-17 = C
18-24 = B
25-29 = A
30 = S 

To Earn the Rank of X: 
1. Defeat the game in Difficult mode
2. Do not use the Bishamon Sword
3. Acquire the Orge Armor
4. Acquire 30 points (an S ranking)