Derig is the old man found in the Grotto. He is the grandfather of Jen, in Ekla, and a friend of Tsorin in Andra.
You can find Derig either by embarking on Ekla's SideQuest (to get rid of the monsters in the UndergroundTunnel), or by speaking to Tsorin who asks you to give him an encrypted message.
Note: Derig may also be found in Ekla if you began the SideQuest and haven't finished it. You will need to return there with Tsorin's note instead of the Grotto.