The events of One Piece take place in a pirate world, and both pirates and marines appear in the series. Two forces that are always at odds with one another. Even though Marines are typically seen as heroes and pirates as villains, the line between good and evil is often difficult to define.
In One Piece, certain individuals have served in both capacities, having defected from the noble marines to the nefarious ranks of the renowned pirates. But what leads some people to abandon their former allies and take up a life of piracy instead? What difficulties do they confront as they learn to balance their new identities?
7 Diez Barrels
Diez Barrels may not be one of the most popular characters in One Piece. However, he is one of the few characters to have held the title of Marine and eventually pirate. Diez Barrels was formerly a highly respected Marine officer and the father of Worst Generation member, X Drake. Unknown to many, Barrels' good works as a Marine inspired his son to become a Marine.
Despite his accomplishments, Barrels eventually left the Marines to become a pirate for unknown reasons. Barrels, a notorious pirate, attempted to sell the Ope Ope no mi to the Marines but was unsuccessful, and Donquioxte Doflamingo killed him.
6 Donquixote Rosinante
Rosinante, the brother of Doflamingo, always had a deep-seated fear of his brother’s violent tendencies and bloodthirsty nature. The former Fleet Admiral Sengoku discovered Rosinante after he and his family lost their Celestial Dragon status, and raised and trained him to become a Marine Commander.
As Doflamingo's blood brother, Rosinante was the best person to go undercover with the Doflamingo Pirates, which he did flawlessly for many years. In the Donquixote Pirates, Rosinante worked under the code name Corazon until Vergo, one of Doflamingo’s subordinates, found out where his true loyalty lay. After being found out, Doflamingo effortlessly murdered his brother in cold blood.
5 Vergo
After his death at the hands of the former warlord Trafalgar Law on Punk Hazard, many of the characters in One Piece remembered Vergo as a compassionate and honorable Marine vice admiral. However, the opposite is true. Vergo was actually a pirate who Doflamingo sent to infiltrate the Marines.
Even as a little child, Vergo acknowledged Doflamingo as his king. He was completely devoted to Doflamingo because he thought of him as a future king who would control the entire world. Because of Doflamingo's faith in Vergo, the latter was tasked with infiltrating the Marines on his behalf. He succeeded admirably, eventually becoming a vice admiral. Unfortunately, Vergo met his end as a Dofalmingo pirate at the hands of Trafalgar Law.
4 Caesar Clown
The owner of the Gas Gas devil fruit, Caesar Clown is a notorious scientist known for creating artificial devil fruits and weapons of mass destruction. After the dissolution of MADS, an illegal scientific research group, Caesar briefly joined Vegapunk and worked as a marine scientist for them. But Caesar abandoned the World Government and joined the Don Quixote Pirates instead.
After receiving criticism for his many unethical and perhaps harmful experiments, Caesar resigned from the Marines. Caesar thought Akainu would enjoy his experiments, but Sengoku was afraid of them. Caesar, when working for the Donquixote Pirates, fabricated numerous "smiles," or artificial devil fruits. Additionally, he constructed weapons of mass destruction, making him a wanted man with a 300,000,000 berries bounty from the World Government.
3 X Drake
X Drake is the captain of the SWORD secret special marine unit. However, the average characters of the One Piece series do not know that. To them, Drake is a former rear marine admiral who defected to become a pirate like his father. The regular characters in the One Piece series are unaware that Drake is the leader of the SWORD secret special marine unit. Their understanding of Drake is that he was once a rear marine admiral but defected to join his father's pirate crew. Drake, having defected, joined the notorious pirate crew known as the Worst Generation, who had bounties of over 100,000,000 berries as of the pre-time-skip Sabaody Archipelago arc for their heads.
Drake, according to common knowledge, left the Marines and eventually became Kaido's subordinate after founding the Drake Pirates. However, viewers of the show are aware that he is still a Marine and that he just appeared to desert in order to go undercover.
2 Shiryu
Prior to joining the Marines, Shiryu was the head jailer at the massive prison at Impel Down. It was within Shiryu's authority as head jailer to employ physical force on prisoners, and he frequently did so. He tortured prisoners for fun until he was thrown in jail himself.
Shiryu, as the former head jailer, was briefly released when the Blackbeard Pirates stormed Impel Down. But instead of calming things down, he joined the Black Beard Pirates and rose through the ranks to become one of the Titanic's ten captains. Shiryu decided that joining the crew was preferable to his other option, which was to be sentenced to death in prison.
1 Aokiji
The ex-admiral of the Navy is a mysterious figure whose true intentions are clouded. Aokiji's presence in the World Government Navy as an admiral was a significant deterrent to pirates. His icy devil-fruit abilities and fierce sense of justice made him a man to be feared.
However, following a tumultuous confrontation with then-admiral Akainu, Aokiji decided to leave the Marines. He made the decision to take over as fleet admiral after Akainu defeated him. Aokiji's decision to join the Blackbeard Pirates as a former admiral was unprecedented. His decision came as a surprise because he had consistently attempted to weaken the pirates' power. His joining the controversial Blackbeard Pirates has caused quite a stir in the One Piece piracy community.