Assassins

Assassins (暗殺者 or 刺客, Ansatsu-sha or Shikaku), are the formal term used throughout the many worlds, and beyond. They are individuals who is trained to kill other people, normally for political or personal reasons. Oftentimes they work as independent agents for hire.

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 * —About Assassins.

Description
The Baroque Works agents were known to have assassins in Misty Peak. Mr. 5 and Ms. Valentine tried to murder Princess Vivi but failed. Mr. 3 tried to kill the Straw Hat Pirates, Vivi, Dorry, and Broggy but also failed.

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, Califa, Blueno, and Kaku were assigned to Water 7 to get the Pluton blueprints and to kill Iceberg. 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. This strongest generation has been dismissed as criminals when their superior, Spandam, framed them of a debacle at the Enies Lobby.

Hyouzou is a mercenary hired by the New Merman 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 doubles as a maid for the Don Quixote Pirates. Violet is also an assassin for the same crew, using her act as a street dancer as a coverup.

The Funk Brothers, Kelly and Bobby are assassins from Mogaro Kingdom.

The Vinsmoke Family is a family of mercenary killers that is active and infamous throughout the Underworld. They are the royal family of the Germa Kingdom, which gives them supreme command of the kingdom's army, Germa 66; through this command, they accept Underworld contracts to fight wars on behalf of paying nations. This is in addition to the Vinsmokes' own national, expansionist campaigns in the North Blue, including one campaign which saw them assassinate the kings of four North Blue nations.

Gotti is the assassin for the Firetank Pirates.

Big Mam prepared to send an assassin to kill her estranged daughter, Lola, once she learned about Lola's whereabouts.

Classifications
There are Classifications for Assassins:

Other Media
Sino Phoenix is a mercenary hired to act as a professional killer. Having always got his kill, his final target in life was Gill Bastar, whose hands he died at instead.

Translation Issues

 * The term koroshi-ya (殺し屋) technically means "professional killer". The more correct kanji term for "assassin" is ansatsu-sha (暗殺者) or shikaku (刺客). Although, the three can be used synonymous.
 * Hyouzou was referred as a "Manslayer" (人斬, Hitokiri) by the New Merman Pirates members, a samurai who specializes in assassinations.

Notes & Trivia

 * Kuro can reportedly kill 50 highly skilled assassins before they notice his presence.
 * Nami, during her eight years of servitude under the Arlong Pirates, she tried many times to kill Arlong, which included hiring assassins to attack him.
 * Daz Bones has not been directly referred to as an assassin, though he shows knowledge of the profession, saying that the first rule of assassination is "pick off the weakest target first".