Get Bishamon Sword:
Fight through all 20 levels of the Dark Realm. Kill all of the monsters on the 20th level, then open the treasure box here to discover 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. Then head inside and claim the prize, a sword with unlimited magic which will kill any non-boss character in a single swipe. A powerful ally, indeed.
Unlock Difficult Mode:
Defeat the game in under three hours to unlock Difficult Mode.
Second Costume:
To unlock Kaede