The world of One Piece is not easily painted in black and white. While its elites like to perpetuate a realm of pirates and heroes, bad and good, the story has revealed a much more complex geography full of individuals and communities beset on their own paths and goals. With that being said, it's not as if people haven't lost their way.
The only truly evil and corrupt people in this anime are those who block the various roads of the world, either for others or even themselves. It's an ideological war that has seen several casualties; but in the presence of some true miracles, namely the Straw Hats, several characters have redeemed themselves.
Usopp
Usopp has had a lot of redemption arcs across the story. As the Straw Hats' resident coward and the weakest crewmember to many, Usopp has had to face and overcome his weaknesses time and again, mostly because he runs away from them so often.
Whether he's trying to hide behind his "diseases," running away from tough fights, or reacting to anything on Thriller Bark, Usopp has been a masterclass in cowardice. There's no better example, however, of Usopp's growth and redemption than his first one-on-one fight in the series. In a battle to save both Nami and her island, Usopp actually had to run towards his fears and put up a fight.
Franky
When the Straw Hats began looking for a shipwright on Water Seven, the last person that any expected to get on their ship was Franky. Franky debuted in the story as the main gang leader of Water Seven who had his men jump Usopp in a brutal mugging. After the Straw Hats sought revenge, he personally went after Luffy. At this point, many were sure that Franky was going to be the arc's main villain.
No one anticipated the long emotional journey ahead, Franky's involvement with helping Nico Robin find herself, or how he'd come to pay back the Straw Hats' stolen money with interest.
Bentham
There are few tropes better in shonen stories than seeing a bad guy change their ways and befriend the heroes. Bentham, aka Mr. 2 Bon Clay, is one of the series' most exciting examples of this. When he first met the Straw Hats, he didn't even realize that they were his enemies, and he quickly befriended themm.
After a grueling fight with Sanji, Bentham was quick to apologize to the Straw Hats by sacrificing himself on Alabasta and getting arrested. He'd go on to do this again in Impel Down when he personally took control of the facility's exit gates and took on Warden Magellan personally. It's as if Eiichiro Oda never wanted him to be a villain in the first place.
Gin
Many may not Gin due to how long ago and unsubstantial his story turned out to be. But for those still in early One Piece, Gin offered one of the most captivating character arcs for a series still trying to find itself. Gin was a commander for the Krieg Pirates who'd bring his captain and crew to the Baratie after Sanji saved him from near starvation.
While Gin expected Don Krieg to be as grateful toward the blonde cook's generosity, he'd quickly come to regret it as Don Krieg immediately tried to take over the ship. Caught between Sanji and his captain, Gin's only act of penance came when he'd sacrifice himself to save Sanji during Don Krieg's poisonous gas attack.
Trafalgar Law
Trafalgar Law was thrust onto a path of darkness and destruction at a young age after witnessing disease and genocide take away his home country of Flevance. This disillusionment would bring him to the Donquixote Family where he'd find the very people who'd help him nurture his hatred and loathing while also ironically the very person who'd save him from them and himself.
That would be Donquixote Rosinante, aka Corazon, who aimed to show Law that there is hope and good people in the world, even if that meant just himself. While Corazon failed to find a doctor that could or would save Law, he showed him the kind of comion that kept him from becoming Doflamingo's kind of pirate.
Saint Donquixote Mjosgard
Few villains in One Piece have been as despicable as the Celestial Dragons. The elite of One Piece's world, these are the ancestors of world leaders who have the birthright to arbitrarily dictate law and order. This has come to make the Celestial Dragons selfish, cowardly, and inhumane, committing atrocities against the lower class just for fun.
Saint Donquixote Mjosgard used to be one of the worst when he first landed on Fishman Island. He had total disdain for the fishmen and only wanted to enslave them. However, his time with Queen Otohime had changed him entirely. Not only would he go on to free all of his slaves, but he also went on to defend and represent Fishman Island during the recent Reverie.
Kyros
Kyros has one of those backstories meant to make people start treating pitbulls better. Born on the streets of Dressrosa, Kyros was a tough and vicious child who knew how to do nothing else but fight and kill. Having seen Kyros' boundless rage, King Riku had him let off steam within his arena, declaring that if he were to win 100 battles that he'd be forgiven for his crimes.
Kyros would easily reach that goal; but because of society's prejudice against him, he'd stay in the colosseum to continue fighting. This would inspire a winning streak into the thousands that would make him a national hero and one of Riku's most trusted guards. That however didn't hold a candle to him giving all of that up to start a family with Riku's daughter, Scarlett.
Nico Robin
Nico Robin was told ever since she was a child that she was unwanted and a demon. The daughter of a revolutionary, Nico Robin was seen even by her foster parents as a troubled child. Her only solace in the world was Ohara's library and its scholars. However, the World Government would come to destroy all of it and label her an enemy to the entire world.
This would spark her decades-long attempt to escape, infiltrating dangerous organizations to steal their knowledge and betraying powerful groups. After years of running, betraying, and fighting, Nico Robin would finally find peace once she ed the Straw Hat Pirates.
Hachi
Hachi's redemption in the story was a major surprise to the fans, despite the character's inherently silly design and demeanor. He was part of the Arlong Pirates, a vicious group of fishmen that aimed to take over the world from the humans. Their first target was Nami's home island, Cocoyasi, which they ceaselessly threatened and taxed, going as far as to kill Nami's mother.
While Arlong was the ringleader, all of the fishmen were immediately despised by the fans for their atrocious behavior. Even though Hachi managed to turn his life around after escaping capture and starting his own takoyaki business, he'd fully redeem himself for what he did to Nami when he took a bullet for Luffy.
Bellamy
No one was expecting Bellamy the Hyena of all people to get a redemption arc. When he debuted, he screamed filler character meant to get one-shot so that the rest of the cast could warm-up for the actual story. He was just that annoying and just that unsubstantial.
Holding his bounty and powers over Luffy's head and making fun of Luffy for just believing in Skypeia, Bellamy was just the type of villain One Piece fans had come to hate by instinct. Nobody knew that he'd go on to become the national hero of Dressrosa or that people would actually cry for him when he regretfully challenged Luffy one more time. This right here is character progression.
