Samurai Warriors: Katana

Enter the Buy/Ability menu. Purchasing all the upgrades for the stat in the indicated level you are in (for example, attack, defense, life, Musou) to unlock another item slot.

Third slot: Musou: Acendancy
Fourth slot: Musou: Swordsman
Fifth slot: Musou: Stealth

Successfully complete all four stories with a "Silver" or higher rank.