Moorinerk said Sidrunerk disappeared while working in the Abyss, after being hired by the Black Cloud Merchants. So you asked Kamorunerk at the Airship Dock, but he didn't know and said you should go see Ondarinerk. Ondarinerk suspected Sidrunerk had been kidnapped by the Steel Rake pirates. When you helped Sidrunerk to safely escape from Steel Rake and returned to Ondarinerk, he expressed his gratitude and rewarded you.