Unlock Chaos Difficulty
Complete all missions with a S rank.
Unlock Fifth Story
Clear all four stories with at least a silver Clear rating.
Unlock Item slots
From the "Buy/Ability" menu, purchasing all stats upgrades for the level you're in. (attack, defense, life, musou etc.)
3rd Item Slot - Max out each stat level in Musou: Acendancy
4th Item Slot - Max out each stat level in Musou: Swordsman
5th Item Slot - Max out each stat level in Musou: Stealth