One Piece is already a story whose 20 year long run is filled with a lot of detail and parallel stories between characters. Though the Straw Hats are the focus, there are countless other adventures going on in the background whose intrigue and importance could rival their own. However, this list is not so much interested in the collective stories of One Piece's modern age but the history that it's built on.
A key component that the series has used to get fans invested into the characters is its affecting use of backstories. The individual adventures of each character carry a lot of detail between them yet each still maintains their own style and tone.
10 Senor Pink
While Franky's battle with Senor Pink initially just seemed like any other Shounen battle, Eiichiro Oda surprised everyone by dropping an intense and tearjerking backstory in the middle of the fight. Pink at one point had fallen in love with a woman named Russian who he started a life with; but she and their son would both see unfortunate accidents.
With his son dead and his wife in a vegetative state, the only thing that could make Russian smile was Senor Pink dressing in their son's old diaper and bonnet. Now adorning this as his regular gear, Senor Pink somehow remained one of Doflamingo's most stoic warriors.
9 Kyros
Kyros could easily fall into a classic, Shounen anime that would do well today. He was once just a street punk in Dressrosa who got into a few fights and even killed a few people. When King Riku got wind of this, he'd send the boy to compete in his coliseum, where prisoners can earn their freedom by winning 100 fights in a row.
Kyros would not only surprisingly do so but also never leave and continue an incredible record spanning hundreds of wins, earning both Riku's and the entire kingdom's respect. Kyros would later join him in his royal guard and even secretly marry Riku's daughter, Scarlett. Things would change once Doflamingo invaded Dressrosa, and Kyros would have to start an entire revolution as a literal, toy soldier.
8 Trafalgar Law
Trafalgar Law's backstory has several parts that could fill several books. His village would be destroyed due the World Government's overzealous treatment of his island's disease. Sick, dying, and wanting some revenge on the world, he tried to join the Donquixote Family only for Doflamingo's brother, Corazon, to try and take him away from the criminal underbelly and find an actual cure.
This would lead to a clash between the brothers over the Op Op Fruit, and Corazon's unfortunate death. Now, seeking vengeance for someone who actually cared for him, Law traveled the world to form his own pirate crew and begin plans to take down Doflamingo.
7 Franky
As odd as it may seem for someone of Franky's aesthetic, his story might appear more homely than most. A good portion of his upbringing was spent learning and being raised by the fishman who built the Gol D. Roger's ship, Tom. He'd be trained alongside Iceburg and craft various wacky and destructive battleships in the process.
When his ship's would be used to frame Tom, Franky would try and stop the sea train taking Tom to Ennies Lobby, only for his body to be nearly destroyed in the process. Spending decades out at sea, scrounging around for scrap to build a new body with, he'd return to Water Seven and start his own gang, the Franky Family, before joining the Straw Hat Pirates.
6 Nami
Nami's tale will have to be one of the sadder ones. As an orphan from a war torn country, Nami would be taken by the kindly Marine, Bellemere, who raised her like her own daughter. Though Bellemere would struggle to provide for her new family, they lived a simple life. Nami herself would often go into the village and steal the various maps and navigation books that would help foster her career.
Arlong's invasion of Cocoyashi Village would of course mean the darker turn of her life, as he'd kill her family and force her to both act as the Arlong Pirates navigator as well as go out stealing treasure throughout the East Blue. Between a rough childhood and her various adventures on her own, Nami's story could have plenty of material.
5 Vinsmoke Sanji
Sanji's backstory has to be one of the most epic ones in the entire story. He was the failed result of Vinsmoke Judge's superhuman breeding and would often fall behind the extreme training of his more inhumane siblings. He would however foster his innate interest for cooking, often serving his developing dishes to his sickly mother.
Judge, however, would become ashamed of his son and force him to wear an iron mask as well as dwell in a prison beneath his castle. It wouldn't be until Germa got into a raucous clash that his sister, Reijuu, would free him and send him off to the East Blue. It's there that Sanji would apprentice at his first cruise ship, get invaded by and stranded with "Red Foot" Zeff, and help build the Baratie.
4 Nico Robin
Nico Robin is one of the most beloved members of the Straw Hat Pirates and much of that has to do with her tragic past. At a young age, Robin's mom would leave her with an abusive foster family, while she was out looking for the Poneglyphs.
Though her day-to-day life was Cinderella-esque (the first half), she would befriend Ohara's archaeologist community and study under them, becoming quite the prodigy in the process. After the Buster Call hit Ohara, Robin would travel the seas, working as a traitor to various organizations while researching to rebuild the efforts of her mother and Ohara's precious researchers.
3 Portgas D. Ace
Portgas D. Ace was the outcast son of Gol D. Roger. After Garp took him in as a baby, he'd leave Ace in the care of some mountain bandits and would occasionally visit him to give him some training. While his family's past would often make him feel cursed, he'd find new purpose in life with his brotherhood with Sabo and Luffy.
When he became old enough to ride the seas by himself, he would rise in infamy as the captain of the Spade Pirates and, later on, as the Second Division Commander of the Whitebeard Pirates. His adventure would near its end when he ventured off to hunt down the traitorous Blackbeard.
2 Kozuki Oden
Kozuki Oden is a fresh story in the One Piece world, but his tale is already one of the greats. He was the wild heir to the Wano kingdom who was banished from the nobility. On his own, his powerful yet attractive personality would garner him followers, as he'd explore some of Wano's more troubled sects.
Having both done some good across Wano as well as amassed a good number of disciples, Oden was allowed to become a daimyo of Kuri, only for him to immediately abandon that life to join the Whitebeard Pirates and, much later, the Roger Pirates. After returning from his grand pirate adventure, he'd clash with Lord Orochi's new rule and his partnership with "Hundred Beast" Kaido.
1 Donquixote Doflamingo
Donquixote Doflamingo's life is the very definition of tragedy. As a child, he was a Celestial Dragon whose family would leave the Holy Land to live a humble life among the peasants. However, the citizens of the world took this as an opportunity to exact revenge against them for all the evil that the Celestial Dragons as a whole have caused.
Doflamingo's family would then live a life on the run, where he and his brother would have to dig through trash for food, and his mother died in a dirty hovel. This would traumatize Doflamingo and have him kill his own father in spite before falling in line with some criminals who'd raise him as an evil master. He'd then follow a path to destroy everything between the regular people that tormented him and the World Nobles that rejected him.