Klabautermann

Klabautermann (クラバウターマン) is said to be a water spirit (or nix) who assists sailors and fishermen on the seas and 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 carrying a wooden hammer. 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 the Skypiea Arc, when Usopp saw it fixing the heavily damaged ship. The sharpshooter of the Straw Hat pirates overheard the Klabautermann telling the Going Merry that it would strive to make it fit for sailing the crew farther. It was spoken of briefly by Franky when he had kidnapped Usopp after he had temporarily left the Straw Hats crew. He is revealed to be a seafaring spirit that aids ships of crews that truly deserve it. It can be said that it was the Klabautermann of the Going Merry that spoke to the Straw Hats at the Going Merry's viking funeral.

Trivia

 * Strangely enough, despite not making much of an appearance, the Klabautermann was ranked as the 49th most popular character in the series (with 46 votes), 2 votes more than the Going Merry herself.
 * Although One Piece is filled with cyborgs, Devil Fruit users and fantastic races (like giants and fishmen), so far, Klabautermann is one of the very few "true" supernatural beings to appear so far.