The Greythorne hearts is a sidequest from Baten Kaitos Origins that is mentioned in the first disc, but can only be accessed once you get to disc 2. To start this Sidequest, go to the old man's house near the orphanage in Sheratan and talk to the Old Man, who will give Sagi the Heart Link. From now on every time Sagi meets a Greythorne that is described as "looking straight into my heart," you can put Salty Water on it to trade bodies. There are people that will tell their secrets to Greythornes, rather than open up to actual people. To complete this sidequest, it is necessary to travel the world and talk to eleven people while in the body of a Greythorne. Be sure to talk to everyone in the same area as well. After completing it, return to the old man to receive the Catfish King's Whiskers.