Its in-game description reads "Lets you steal enemy's items in battle." It has the "Steal" ability which means there's a chance that one of Monica's projectile will steal a item from the enemy. Like all her armbands, you can charge it, remember the closer you are and the longer you charge, the stronger will be the magic projectile. It can be useful but do not let the enemy get too close to you. You can find it in dungeons' chests, you can also invent it, but you can buy it at Starlight Weapons in Balance Valley's future for 700 Gildas when you get to the Chapter 3.