The Klabautermann is one of the most central and mystical elements of Eiichiro Oda's One Piece, derived from oceanic folklore, with unique importance attached to the series narrative. According to One Piece, a Klabautermann refers to the form of a soul which a ship may take only when it is deeply loved and cherished by its crew. This aspect creates an emotional layer in the relationships shared between the characters and their ships in the series.
A Klabautermann in One Piece is depicted as a ghostly humanoid child, typically wearing a raincoat and shoes of a sailor, along with a little wooden hammer. Obviously, this appearance draws from folklore, which inspired Oda, as there the Klabautermann are described as small sailors or water goblins. Particularly, the hammer corresponds well since it alludes to the Klabautermann's function of keeping the ship he inhabits in good order and repairing it if necessary.
Probably the best Klabautermann appearance in One Piece is on the Going Merry, the first ship of the Straw Hat Pirates. During the Water 7 arc, Usopp claims to have seen some tiny figure fixing the ship at night. This was later confirmed by Franky, who describes to the crew what a Klabautermann is, saying that it rarely and importantly appears.
In the case of One Piece, a Klabautermann would have the following functions for the narrative: first, it underlines how deep the relationship between a crew and their ship can be, elevating said ship from being mere transportation to the level of a character; second, often foreshadowing major events or decisions relating to the ship in question, as seen in the eventual fate of the Going Merry.
The Klabautermann in One Piece are referred to as abilities that preserve the folkloric origins of their being. They possess experience in most watercraft, which they can use to aid the crew in dangerous situations and to perform maintenance on the ship. The Klabautermann of Going Merry was actively repairing the damage to the ship; thus, it makes a tangible kind of personification of the ship's will to sail with its beloved crew.
Klabautermann's concept in One Piece also links into broader themes within the series, particularly the idea of inherited will. Just as characters can inherit the will of those who came before them, similarly, ships in One Piece could gain their own Will and Spirit, which the Klabautermann presents best.
Worth noting is that in One Piece, a Klabautermann is an incredibly uncommon sight. Going Merry's Klabautermann is one of the few actually shown in the series, besides highlighting how strange it is that the Straw Hats bond with their first ship. This fact makes the moments when a Klabautermann appears all the more emotional; such moments become really memorable both for the characters and the readers.