The cabin boy of the Lady Zay can be found on the pirate ship at the start of Lunar Diplomacy and then on Lunar Isle, west of the bank, after.
During Lunar Diplomacy, the cabin boy says that he was asked by a girl on Lunar Isle to put the jinx symbols on the ship. After being talked to by the player, he provides an emerald lantern lens and a bullseye lantern frame, which are combined to make the emerald lantern. He will also give the player a vague hint as to where the symbols are.
The cabin boy is on the isle to confront the girl. The Moon Clan girl, however, apparently puts another spell on him, as he acts strangely when talked to on the isle.