Eiichiro Oda's One Piece is the story of Monkey D. Luffy, a young boy who aims to become the King of the Pirates in an era known as the Great Age of Piracy. On his journey, Luffy acquires some of the most amazing crewmates and comes across some incredible people who Oda certainly adds quite a lot of depth to.
Often, characters end up forming deep relationships with Luffy, who is known to hold the power to change others around him and affect them in positive ways. Over the course of over 1,000 One Piece chapters, fans have seen countless characters change, some in more profound ways than the others.
8 Nami
Nami is a member of the Straw Hat Pirates and serves the role of the navigator of the crew. Originally, Nami met Luffy in Orange Town and made it quite clear that she despised pirates. Having had her whole life turned upside down by them, Nami's hate was certainly understandable.
Amazingly, by the end of the Arlong Park arc, Nami had already become a pirate herself and realized that some pirates, such as Luffy, are actually quite kind. Since then, Nami has dedicated everything to the crew just for them to fulfill their dreams.
7 Chopper
Just like Nami, Chopper is yet another member of the crew who changed drastically as the story went on. Being born a blue-nosed reindeer, Chopper was cast out by his own kind at a young age. Then, after eating his human-human fruit, the humans hated him too.
Chopper was called a monster and this made him keep a distance from all those who would hurt him, at least until Luffy came along. Luffy and the crew filled the hole in Chopper's heart and, now, he is even willing to become a monster for the sake of those he loves.
6 Bon Clay
A member of the Baroque Works organization, Bon Clay was an enemy of the Straw Hat Pirates throughout the majority of the Alabasta Saga. He created a lot of problems for the crew in the arc and even fueled the fires of the civil war in the country.
Bon Clay also attempted to kill Vivi but ended up being stopped by Sanji. At the end of the day, however, Sanji's act of kindness touched him and he sacrificed himself for the cause of the Straw Hat Pirates, shocking as it might be. During the Impel Down arc, he sacrificed himself once again in order to let everyone else escape from the prison.
5 Crocodile
The leader of the Baroque Works, Crocodile was one of the key figures in the world of One Piece, especially before the time skip. Being one of the Shichibukai, Crocodile had great freedom on the seas and he used it to get a grip on the country of Alabasta in order to find the Ancient Weapon Pluton.
Unsurprisingly, Crocodile was defeated by Luffy and was locked up in Impel Down by the Marines. After being freed, however, he appears to have changed drastically and even saved Luffy and Ace during the Marineford arc.
4 Robin
Another former Baroque Works member, Nico Robin worked with Crocodile as his number 2 during the Alabasta arc. Prior to working with him, she was a part of countless other pirate institutions that she betrayed to get closer to her goal.
However, after her life was saved by Luffy, Robin couldn't help but feel like she belonged with the crew. She even went as far as to sacrifice her life for the crew in Water 7. Eventually, she was rescued by Luffy and the group and she's devoted herself to making sure Luffy becomes the Pirate King ever since.
3 Roronoa Zoro
The swordsman of the crew, Roronoa Zoro was already infamous in the East Blue as the Pirate Hunter before he met Luffy. Although he agreed to join Luffy when the latter saved him from execution, Zoro made it quite clear that his goal is to become the World's Strongest Swordsman and he won't let even Luffy come in between him and his dream.
Shockingly, however, by the time of the Thriller Bark arc, Zoro was ready to sacrifice his ambition for Luffy's sake. At the same time, he even bowed down before Mihawk to make sure he was strong enough to make Luffy the King.
2 Donquixote Mjosgard
One of the despicable Celestial Dragons, Saint Mjosgard was introduced to fans in the Fishman Island arc. Just like the others of his kind, he was quite heartless, however, Otohime saving his life and ensuring a safe journey back to Mariejois changed him.
When Shirahoshi was attacked by Charloss during the Reverie arc, it was Mjosgard who saved her and set her free. For a Celestial Dragon to go against his own kind is certainly an incredible thing to do.
1 Trafalgar Law
Trafalgar Law was the last know survivor of the tragedy of Flevance, also known as the White City. Although Law lived a happy life at one point, the genocide that he witnessed changed him for the worse and he simply wanted to destroy the world as much as possible for his remaining 3-year lifespan.
Thanks to Corazon, however, Law was saved from being consumed by this monstrous mentality and, at the same time, he was given a new life with the Ope Ope no Mi that the former stole. Law continued to live his for Corazon from that point until Doflamingo was defeated for good. Currently, he aims to find the answers to the uncertain fate of the D.