Klabautermann

Klabautermann (クラバウターマン, Kurabautaaman) is a water sprite (or nix) who assists sailors and fishermen on the Baltic Sea in their duties. He is a merry and diligent creature, with an expert understanding of most watercraft, and an unsurpassed musical talent. He also rescues sailors washed overboard. He first appeared in Chapter 254 and Episode 167.

His image is of a small sailor in yellow with a tobacco pipe and woolen sailor's cap. This likeness is carved and attached to the mast as a symbol of good luck.

Despite the positive attributes, there is one omen associated with his presence: no member of a living ship shall ever set eyes on him. He only ever becomes visible to the crew of a doomed ship. This is shown to be true in One Piece, as Usopp saw the spirit fixing Going Merry back in Skypiea, and in the end, due to various events occurring, Going Merry was broken beyond repair and had to receive her Viking Funeral.

Story
The Klabautermann of the Going Merry was first seen during Skypiea arc, when Usopp saw it fixing the heavily damaged ship.

Trivia

 * Strangely enough, despite not making much of an appearance, the Klabutermann was ranked as the 49th most popular character in the series (with 46 votes), 2 votes more than the Going Merry herself.
 * A sidenote: Klabautermann may not have been the one to fix the Going Merry during the Skypiea arc, as it is said that it has an expert understanding of most watercraft, while the Going Merry was clearly fixed by a rookie. It could've been Chopper, as he desperately wanted to prove himself to be a contributing member of the crew and he was very excited about finding out Usopp's opinion about the intention's of the person who fixed the ship.