The world of One Piece is mainly divided into two sides: Marines and Pirates. The Marines are a massive organization that ensures that pirates and other criminals stay in line and don't cause any problems for the people of the world. There have been quite a few Marines who have joined pirate crews in order to gain more information.
Likewise, there have also been pirates who joined the ranks of the Marines to keep their crew members informed. However, there has been no pirate who has completely sided with the Marines and fought for justice. With that in mind, series fans may be interested to explore some of the most notable One Piece pirates who would go on to become excellent Marines.
1 Roronoa Zoro
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Before joining the Straw Hat Pirates, Zoro was a bounty hunter who caught pirates and handed them over to the authorities. He became extremely famous because of his accomplishments, and pirates throughout East Blue started to fear him.
If Zoro were to become a Marine, he would unquestionably be a good one. Zoro has shown that he cares about the well-being of other people, and for that, he is willing to risk his own life. According to Oda, if Zoro wasn't a pirate, he would be a police officer, which further proves that he is an excellent candidate.
2 Shanks
Shanks is one of the most powerful characters in One Piece, yet there are not many people who are as easygoing as him. He is among the few characters who are respected by both pirates and Marines.
Above all, Shanks is adored by his crew members and the common people. Shanks' leadership has helped his crew cement their place among the elite. These are the qualities that one would want in a Marine. If Shanks were to become the Fleet Admiral, the entire organization would perform better, and be more harmonious.
3 Whitebeard
Whitebeard was Roger's greatest rival, and he was considered to be the man closest to the One Piece. He had the power to destroy the entire world, but he avoided engaging in any major conflicts because he was content sailing with his family.
Whitebeard didn't consider his crewmates underlings, instead referring to them as his sons. A man of such compassion and morality would be a blessing for any organization, including the Marines. With Whitebeard at the helm, the Marines wouldn't have to fear anyone, and they would love working under him.
4 Trafalgar Law
Trafalgar Law is a member of the Worst Generation and one of the strongest pirates in One Piece. He made his debut in the Sabaody Archipelago arc, during which he showcased his immense strength. Law possesses the Ope Ope no Mi, a Paramecia-type Devil Fruit that allows him to create ROOMs in which he controls everything. From his involvement in the Summit War, it was evident that Law would become an important character, and he did exactly that.
He went on to play a crucial role in several arcs after the time skip. Law allied with the Straw Hat Pirates, and together they defeated Doflamingo, Caesar, Big Mom, and Kaido. Law has the rare quality of being able to work with other people. Considering the Marines are sent on missions together, Law would have no trouble getting along with them.
5 Portgas D. Ace
Portgas D. Ace had a very rough childhood. All people mocked him for being the son of Gol D. Roger, and this caused Ace to question his existence. Eventually, Ace met his two brothers, and he got over this mental issue. Garp actually wanted Ace to become a Marine, but the latter refused to do so.
If Ace had agreed to go along with Garp's choice, he would have become an excellent Marine. Ace was powerful yet kind. Ace was a well-mannered character with a strong sense of duty, as evidenced by the fact that he wanted to capture Blackbeard because he was in his division. Ace also cared a lot about his friends, and if one of them were to be harmed, he wouldn't allow the culprit to go unpunished.
6 Chopper
Chopper is an adorable reindeer who ate the Hito Hito no Mi. The Devil Fruit gave Chopper the ability to talk and think like a normal human being. After being cast out by his fellow reindeer, Chopper was taken in by Hiriluk, who treated him like his own son. Eventually, Chopper joined the Straw Hat Pirates when they visited Drum Island.
Despite being a pirate, Chopper is unbelievably benign. He goes out of his way to help people and treat their injuries, often putting his life at risk. This makes Chopper an oddity in the dark world of pirates, so if he became a Marine, he would be adored by everyone.
7 Charlotte Katakuri
Charlotte Katakuri is a member of the Big Mom Pirates. He was one of the main antagonists of the Whole Cake Island arc, and he was Luffy's main opponent. Unlike most of Luffy's previous enemies, Katakuri was not malignant.
Katakuri was very caring toward his family, and he used his strength to protect them from any possible harm. His desire to keep his family safe is a rare trait among pirates, who are known for infighting and betrayals. If he were to become a Marine, his qualities would undoubtedly make him popular with the lower-ranked Marines, as they are mostly treated as dispensable tools by their superiors.