There are seven different medals that can be obtained in Skyward Sword. If you keep a medal in your Adventure Pouch it will grant you beneficial effects. They are completely optional, but are helpful accessories.
Medals you are not using can be stored in the Item Check.
The Cursed Medal is the only medal that is both beneficial and harmful. It will increase your chances of finding both rupees and treasure, but it also makes you unable to open your Adventure Pouch. You’ll have to deposit this medal in the Item Check in order to equip a shield or use a potion.
The Cursed Medal is an optional reward from Batreaux after you find 30 Gratitude Crystals. A treasure chest will appear in Batreaux’s house after you show him 30 Gratitude Crystals. Batreaux will warn you about the dangers of the medal, but you can take it anyway. If you’d rather not bother with depositing it in the Item Check, you can leave the chest unopened.
The Heart Medal will cause hearts to appear more frequently in the field. Carrying this medal with you is the only way to make hearts appear at all if you are playing a Hero Mode save file.
The Heart Medal is found in a Goddess Cube treasure chest. You must activate a Goddess Cube in the Lanayru Desert to make this treasure chest obtainable.