What is saddest one piece backstory?

One of the saddest backstories in One Piece is the story of Nico Robin.

Nico Robin was born on the island of Ohara, which was known for its scholars and vast library. However, the World Government viewed the scholars of Ohara as a threat and ordered their extermination.

When Robin was just a child, the World Government sent the military to attack Ohara and destroy the library, killing her mother and other scholars in front of her. She was then taken in by a group of archaeologists, but they were eventually killed as well, leaving Robin alone and constantly on the run as a criminal.

Throughout the series, Robin struggles with the trauma of losing everyone she has ever loved and living a life constantly on the run. Her backstory is often cited as one of the most heartbreaking in the series.