The Master's Circle is a ring panel obtained in Kingdom Hearts 358/2 Days upon redeeming 358 Mission Crowns at the Moogle. The number of Mission Crowns that must be obtained is an underlined reference to the number of days that Roxas is in Organization XIII. It gives 100% protection against elements, but prevents you from healing when you have low health and reduces your health each time you miss with an attack.
You can take this item two ways. One side is the ring is a horrible item not being able to heal and lose HP when you miss. The other side is it will help you develop useful skills for future kingdom hearts games. The not being able to heal side could be there to teach you do something before it's to late. In a battle It teaches you to heal before your HP gets to low and if you don't you pay the price. The lose HP when you miss teaches you to not waste excess energy. Don't waste your time fighting thin air but spend it beating up heartless