Bounty hunters are people who make a living out of catching criminals for the bounty on their head.
Unlike mercenaries or assassins, who are hired or commissioned by the government to hunt down criminals, bounty hunters and bounty hunter groups operate completely independently, working outside the law to capture criminals. It is typically common practice to capture criminals alive rather than kill them, as Igaram stated that they could lose up to 30% of the bounty if they killed their target. While most bounty hunters operate for money, some seek the fame that results from taking down infamous pirates, such as Jean Ango.
Successful bounty hunters will often inevitably become well-known, especially by wanted criminals, and even normal civilians may become afraid of them. Since they operate outside the law, bounty hunters have been known to partake in criminal activity, although being labeled as criminals typically prevents them from bounty hunting again since they can no longer deal with the government. Even though bounty hunters may not be labeled as criminals, they seem to be held in disdain by law enforcement like the Marines, as Tashigi considers them to be no better than pirates.
In rare cases, organizations other than the World Government can place bounties on people's heads. While most simply hire mercenaries, Donquixote Doflamingo used his status as the king of Dressrosa in order to place a star bounty system on the Straw Hat Pirates and their allies when their actions against him were brought to light, enticing the people of Dressrosa to capture them and bring them to Doflamingo for a reward.
Several years ago, the Shichibukai Sir Crocodile founded the crime syndicate Baroque Works. Putting on an act of hunting pirates himself, Crocodile recruited hundreds of bounty hunters to join the organization and work to build an ideal nation. The bounty hunters worked to capture pirates and criminals to aid Baroque Works' operations, but Crocodile really intended for the organization to take over the kingdom of Alabasta. Daz Bones, one of the West Blue's most infamous bounty hunters, joined Baroque Works and became the high-ranking Mr. 1.
After getting lost while traveling the East Blue, Roronoa Zoro turned to bounty hunting in order to make a living. His exploits earned him a great reputation and the title "Pirate Hunter", and he worked with fellow bounty hunters Johnny and Yosaku for a time. Zoro was recruited to join Baroque Works, but killed the agent who recruited him after being denied leadership of the organization. Zoro would later join the pirate Monkey D. Luffy after being arrested by the Marines on Shells Town to form the Straw Hat Pirates. Johnny and Yosaku continued bounty hunting for a time, but after a succession of failures, they decided to become fishermen instead.
Baroque Works would meet Zoro again when the Straw Hats came to Whiskey Peak. There, the agents knocked the pirates out and planned to turn Luffy in for his bounty. However, they were confronted by Zoro, and despite numbering 100 strong, the agents were all defeated by him. Baroque Works would later be dismantled by the Straw Hats and Marines with the help of Miss Wednesday and Mr. 8, who had gone undercover in the agency to save Alabasta. Some agents were arrested, while others pursued new lives afterwards.
The Franky Family operated on Water 7 as both bounty hunters and ship dismantlers. When Franky was captured by the World Government agency CP9, the Family worked with the Straw Hats to invade Enies Lobby and rescue him. After the invasion, Franky joined the Straw Hats, and the Family members found other jobs as they ceased to be outcasts in Water 7.
Many bounty hunters were stationed on Sabaody Archipelago due to the large numbers of pirates there. Several waves of them attempted to capture the Straw Hats, but were unsuccessful. However, two of them, Kairiken and Minoruba, alerted kidnappers to the presence of Keimi the mermaid with the Straw Hats, causing them to capture her.
After Luffy freed several Level 6 prisoners during his raid on Impel Down, Jean Ango worked to capture several of the prisoners, and he also set his sights on capturing Luffy and his escape party. He came into contact with Luffy while fighting in the Corrida Colosseum on Dressrosa, and attacked him, but was ultimately defeated. Later, Doflamingo unveiled his "star bounty" system and placed bounties on the heads of the Straw Hats and several of their allies. This enticed several of Dressrosa's gladiators and criminals to go after the bounties, but they later turned against Doflamingo and helped Luffy after Doflamingo moved to kill everyone on Dressrosa.
Scorpion was a bounty hunter who retired to raise his sons Chip and Dip in the Badland in Alabasta. However, as life grew tough, he decided to hunt down Portgas D. Ace, but was unsuccessful as Ace saved him instead.
Shuraiya Bascùd was a bounty hunter for eight years, whose goal was to kill the pirate Gasparde after he destroyed his hometown. He was unsuccessful, but Gasparde was defeated by the Straw Hats. Shuraiya then learned that his sister Adelle Bascùd was still alive and decided to spend his life with her.
The bounty hunting family the Accino Family trapped the Straw Hats in their territory Hyokaido. Using the terrain to their advantage, they successfully captured most of the Straw Hats for their leader Don Accino's birthday present. However, the Straw Hats managed to fight back, defeat the Accino family, and escape.
Gardoa is a bounty hunter who uses the bounties he earns to buy shiny spears. He entered the Corrida Colosseum competition on Dressrosa and intended to take down Cavendish for his bounty, but Cavendish easily defeated him.
- Bounty hunter — Wikipedia article on bounty hunters.