Jonah is located directly west of the Beach Shrine on Sentinel Island. He gives you the quest The Old Ghost and the Sea

Jonah is a ghost now but in the past he was a sail and a fisherman.At the time he was a sailor they hunted down a demon whale Mocha Dick. The animal sank his ship but Jonah´s ghost made it to Sentinel Island. Now even after death he still wants a precious item from the whale which he claims he lost.

You can mind read him for 700 exp to learn that Vacca picked up an item Laiken dropped that is located in Vacca's Cave near his books.