Mercenary

A mercenary is a person who takes part in an armed conflict who is not a national or a party to the conflict and is "motivated to take part in the hostilities by the desire for private gain".

Overview
Mercenaries are individuals who join a certain organization solely for personal gains, mainly monetary profits. Because of this, they are not bound to their employers, and once a transaction is complete, they are not obligated to follow any further orders, and may even leave or betray their contractors in between if they desire. If the want-to-be employer cannot (or will not) accommodate the mercenary's fees, then they can easily refuse to serve said employer. Even if hired, they may not be fully committed to the employer's orders, and may end up doing something out of the given task.

Their services provided can have a broad variety, such as bodyguards, assassins, enforcers, or high-ranking soldiers.

In One Piece
The first mercenaries to appear were the Koala Mercenaries. They were a trio of bodyguards hired by Crocodile to act as bouncers of the Rain Dinners casino. They tried to stop Monkey D. Luffy from running wildly in the casino, only for him to run through them without noticing their presence, sending them crashing into a slot machine. Upon Crocodile's dismissal from the Shichibukai and the shutdown of Rain Dinners, the mercenaries lost their job as bodyguards.

Hyouzou was a mercenary of Fishman Island. He was considered to be the most powerful swordsman of island, and was hired by Hody Jones to act as an assassin and officer for the New Fishman Pirates. Indeed, Hyouzou fought not for the crew's misanthropic ideals, but for money; he refused to join the Arlong Pirates a decade ago because Arlong had issues with paying Hyouzou's high prices. After his overdose on the Energy Steroids and imprisonment in the Ryugu Palace dungeons, Hyouzou lost all of his natural strength and position as a mercenary.

The Yeti Cool Brothers are mercenaries hired by Caesar Clown to act as enforcers and killers for him in Punk Hazard. They do sometimes kill their targets from going overboard, but they nevertheless act to accomplish a given task. The two were ultimately defeated by Franky and Trafalgar D. Water Law, leaving their overall fate unknown when Shinokuni flooded the entire island.

Hajrudin, known as the "Pirate Mercenary", was employed under the Buggy's Delivery society, alongside four other giants. However, after the Dressrosa incident, the five giant mercenaries quit the society for Hajrudin to join the Straw Hat Grand Fleet as the 6th division's commander, much to Buggy's fury.

The Germa 66 is an underworld military force that offers its services as a mercenary group. Originating from the Germa Kingdom, the group directly led by the kingdom's royal family, the Vinsmoke Family, members whom enter the front-line as commanders. Their power is infamous within the underworld, as guided by Vinsmoke Ichiji and Vinsmoke Niji, they were able to end a two-year civil war on Broc Coli Island within a mere four hours. The services that Germa 66 are strictly business, having no attachment to ideals nor loyalty for their clienteles, as Vinsmoke Yonji claimed they would have earned more money if they took up both sides of the Broc Coli civil war, Niji stating that they simply serve the side who gives the better offer, and Ichiji even coldly killing a heartbroken soldier, disregarding the desperate man's cries over his slaughtered family.

Filler-Exclusive Mercenaries
Golass was a mercenary hired by El Drago to help find Woonan's alleged lost treasure. However, he dislikes fighting for money, and is apparently ashamed to have done so. He lost to Roronoa Zoro, who saw through his true character.

Eric the Whirlwind was a mercenary hired by Nelson Royale of the Marines to help find the fabled Sennenryu. Later, it turns out that Eric was using Nelson to find the Sennenryu for himself, and ruthlessly disposed of his contractor once deeming him no longer useful. Eric himself was later killed when he tried to exact his revenge against the Straw Hat Pirates.

In Other Media
Sino Phoenix was a mercenary who was hired to kill Gill Bastar. He was a hit-man who always got his kill and disregards collateral and civilian damage, and would work for the police if he wanted. Ultimately, he failed and died by Gill's skills.