Kalm Traveler
Check the top floor of the rightmost building in Kalm. There you'll find a man who's completely content with the world. However, once Meteor is summoned and the WEAPONs are attacking, the man will become discountented. At this time he'll begin to searching for three items: the Guide Book, Earth Harp, and Desert Rose. Bringing him any one of these items will prompt him to reward the party.
The Guide Book is located in the Underwater Reactor, but it's not in a treasure chest. You'll hav eto Morph a creature called the Chost Ship to pick up the item. It can usually be found in the tunnels along the ocean floor. Take the item back to the traveler and he'll give you the Underwater Materia. This item eliminates the 20 minute timer in the fight against the Emerald WEAPON.
The Earth Harp and the Desert Rose are prizes for defeatring the two new weapons. You'll receive the Earth Harp if you can defeat the Emerald WEAPON. If you beet the Ruby WEAPON you'll receive the Desert Rose. Take the items back to the traveler and he'll give you the Master Summon, Master Magic, and Master Command Materia. In exchange for the Desert Rose, the traveler will give you a Gold Chocobo.