The Medium Wallet is the second wallet Link can acquire in Skyward Sword. It can carry up to 500 rupees.
How to get it
The Medium Wallet can be acquired by collecting five Gratitude Crystals and showing them to Batreaux. He will reward Link with the Medium Wallet.
The Medium Wallet can be upgraded to the Big Wallet, with a maximum rupee capacity of 1000. This can be acquired by giving Batreaux 30 Gratitude Crystals and being given the Big Wallet in return.