Supposedly, a vest that belonged to a demon.
Description
The Demon's Vest is a mid to endgame light armor piece in Final Fantasy IX that everyone but Steiner and Freya can wear. It teaches Auto-Potion, which allows the character to heal themselves with a Potion when an enemy hits them; Devil Killer, which improves damage against demon type enemies; and Locomotion, which inoculates against Stop. The vest gives a bonus to the wearer's Magic when they level up with it equipped and halves Shadow damage.
Obtain[]
The Demon's Vest is stolen from Meltigemini, the boss at Mount Gulug. Afterward, it is bought from Daguerreo (before entering the Shimmering Island portal), Ipsen's Castle, and Bran Bal. It is also found in Terra 
.
Mechanics[]
The Demon's Vest provides 31 Defense, mitigating damage from physical attacks. The vest gives a +1 bonus to the wearer's Magic when they level up with it equipped. When enough "bonuses" upon level up from gear have been accrued, the character gains a permanent stat increase. Magic factors into spells' damage calculations and for the character's MP growth.
The vest halves Shadow damage to the wearer and the effect does not stack with other Shadow-halving gear. If the character also wears a Pumice Piece that absorbs Shadow, they absorb half damage. If the character also wears a Maiden Prayer that weakens them to Shadow, they take ~75% damage from Shadow attacks.
Use[]
The Demon's Vest is the best light body armor for world map exploration on a flying transport, Ipsen's Castle, and Earth Shrine, until the player comes across the superior Rubber Suit and Minerva's Plate for female characters. Demon's Vest continues to be the best body armor for Zidane and Amarant for Pandemonium; the endgame light armor pieces will be available afterward.
The player has likely mastered many or all abilities from Demon's Vest already. Auto-Potion is not as good as Auto-Regen, but Devil Killer is good for the random encounters in Terra and Pandemonium. Locomotion is good to master as Stop is a hazardous status abnormality to contract.
The Demon's Vest halves Shadow damage to the wearer. This would help against Doomsday used by Ozma and Hades or Vivi, though Shadow-absorption would be preferable, as well as against Mistodons' Mist attack.
The Demon's Vest has a level up bonus for Magic. It is not advised to level-grind unless the player is stuck in a boss or a dungeon; level grinding and min-maxing are unnecessary in a normal playthrough even when preparing for superbosses. The best stat boost gear will be available in the end of the game, so for best results one would need to limit their EXP intake. If Zidane and Amarant do not need to master the abilities from Demon's Vest, they may opt to continue wearing the preceding Power Vest for its Strength +2 level up bonus, though this is inconsequential for boss encounters who forgo EXP.
The guest characters from Tantalus can also wear the Demon's Vest though the player cannot equip them with it in a normal playthrough.