Assassin

An assassin (暗殺者) is a person who is trained to kill other people, normally for political or personal reasons. Oftentimes they work as independent agents for hire.

Known Assassins
The Baroque Works agents were known to have assassins in Whiskey Peak. Mr. 5 and Miss Valentine tried to murder Princess Vivi but failed. Mr. 3 tried to kill the Straw Hat Pirates, Vivi, Dorry, and Brogy.

Nico Robin claimed that she is an assassin when Usopp interviewed her upon her entry into the Straw Hat crew, but her targets and reasons have yet to be revealed. However, she was prepared to kill Crocodile since he recruited her into Baroque Works, and tried it when he declared that he was going to kill her, though her attempt ultimately failed.

The CP9 are the first assassins in the series to work for the World Government and they are the only known World Government assassins. Lucci, Kalifa, Blueno, and Kaku were assigned to Water 7 to get the Pluton blueprints and kill Iceburg. Meanwhile, Jabra, Fukuro, and Kumadori were sent to an unnamed town to assassinate three key Revolutionary leaders (Dragon not included). However, they ended up killing twenty extra people thanks to Fukuro revealing his team's presence and their intentions to the town.

Hyouzou is a mercenary hired by the New Fishman Pirates to become an assassin of the crew. His profession came to an end when he overdosed on Energy Steroids, which eventually stripped him of all of his natural prowess, rendering him a frail and weak old man, and the fact that he was arrested and put in the prison of the Ryugu Palace.

The Yeti Cool Brothers are Caesar Clown's enforcers tasked to search for the Straw Hat Pirates, hunt them down and extinguish them with extreme prejudice.

Baby 5 is an assassin who works for Donquixote Doflamingo.