Once you have completed the events in the Underwater City in the past. Return to Krage in the present, and sail south through the river, following it until you can land west. Here you will find the Medal King's Castle. Inside the Medal King will reward you with the following prizes based on the number of Tiny Medals you have collected.

Unlockable	How to unlock
Blue Print	75 Tiny Medals.
Bolero	95 Tiny Medals.
Metal King Shield	90 Tiny Medals.
Miracle Sword	65 Tiny Medals.
Platinum King Heart	110 Tiny Medals.
Poison Dagger	45 Tiny Medals.
Sacrifice Ring	58 Tiny Medals.
Sage Rock	83 Tiny Medals.
Shard	100 Tiny Medals.
Ultimate Whip	105 Tiny Medals.
Wind Staff	50 Tiny Medals.