To unlock this, obtain every piece of torso gear at some point in the game. You do not need to keep them after acquiring them. There are three pieces of armor that are drops and will require some grinding to get.
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Overalls - Potos
Kung Fu Suit - Pandora
Chain Vest - Kippo Village
Kung Fu Dress - Kippo Village
Minor Robe - Dwarf Village
Spiky Suit - Dwarf Village
Fancy Overalls - Mantago
Chest Guard - Kakkara
Golden Vest - Kakkara
Ruby Vest - Southtown
Tiger Suit - Northtown
Tiger Bikini - Northtown
Magical Armor - Mandala
Tortoise Mail - Tasnica
Flower Suit - Gold City
Battle Suit - Gold City
Vest Guard - Neko (Mana Palace)
Dropped
Vampire Cape - Fiend Head (Mana Fortress)
Power Suit - Whimper (MF)
Faerie Cloak - Master Ninja (MF)