Klabautermann

Klabautermann 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.

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 ship blessed by his presence shall ever set eyes on him. He only ever becomes visible to the crew of a doomed ship.

Story
The Klabautermann of the Going Merry was first seen during Skypiea arc Reference:

Strangely enough, it was ranked as the 49th most popular character in One Piece (with 46 votes), 2 votes more than the Going Merry herself.