Assassin

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

Assassins in One Piece
The Baroque Works agents were known to have assassins in Whiskey Peak. Mr. 5 and Miss Valentine tried to kill 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, but judging by her past story, it was unknown who she assassinated. However, she did try to kill Crocodile because he was going to kill her.

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, Jyabura, Fukuro, and Kumadori were sent to an unnamed town to assassinate three key Revolutionaries leaders (Dragon not included). However, they ended up killing twenty extra people thanks to Fukuro.

Hyozo is a mercenary hired by the New Fishman Pirates to become an assassin of the crew.