Upon visiting Laugh Tale, the Roger Pirates found the One Piece and discovered the major secrets of the world. Thus, they learned everything about the Void Century, a mysterious blank period in history during which 20 realms formed the Ancient Alliance to fight and overthrow the Ancient Kingdom.
The Twenty Kings then went to live in Mary Geoise, becoming Celestial Dragons and establishing the World Government. All mentions of the Ancient Kingdom have been erased from history, with studies on the subject being banned. Clearly, most of the people who knew the true history are dead, as the Void Century dates back to 800 years ago.
The Ohara scholars were killed for researching the subject. Gol D. Roger and Oden Kozuki learned the truth after visiting Laugh Tale, but they eventually died. Still, there are some characters who are alive and know the true history.
Imu and the Five Elders won't be included in this list, as it's already common knowledge that they know everything about the Void Century.
Disclaimer: This article contains major spoilers from the One Piece manga up to Chapter 1086. Some aspects of this list are subjective and reflect the opinions of the writer.
Five non-World Government characters who know the true history of One Piece, ranked from the most obvious to the most unexpected
5) Silvers Rayleigh
Rayleigh is the former right-hand man of Gol D. Roger, the Pirate King. A man feared as the "Dark King" for his tremendous Haki powers and swordsmanship, Rayleigh is the old-generation version of Roronoa Zoro.
The second-in-command of the Roger Pirates, Rayleigh was the Pirate King's strongest and most trusted man. Unsurprisingly, Garp considered the "Dark King" a legend of the same caliber as Whitebeard, while the author likened his Conqueror's Haki to that of Shanks.
As Roger's closest associate, Rayleigh visited Laugh Tale with him, learning all the secrets of the One Piece world. The two really turned the globe upside down together, like they promised to do in their first meeting.
Many years later, an aged Rayleigh met the Straw Hats, but he didn't reveal them anything and instead encouraged them to have their own adventure.
4) Scopper Gaban
Based on the clear parallel between the Roger Pirates and the Straw Hat Pirates, Gol D. Roger and Silvers Rayleigh are the old-generation counterparts of Luffy and Zoro. This implies that Scopper Gaban is the equivalent of Sanji.
As such, Gaban was the third strongest individual in the crew after the captain and his right-hand man. However, after Oden Kozuki joined the Roger Pirates in their last trip, Gaban may have dropped to fourth place in the group's strength ranking.
Regardless, he was an extraordinary pirate, as a major member of the crew that conquered the Grand Line. Undoubtedly, Gaban witnessed what the One Piece truly was and discovered the events that took place during the blank period of the Void Century.
3) Crocus
Crocus has been the lighthouse keeper at Twin Cape for a long time. Twenty-six years before One Piece's current narration, Crocus joined Roger's crew to find out what happened to the Rumbar Pirates, who once stopped by his place and left the young whale Laboon but never came back to retrieve it.
An exceptional doctor, Crocus was able to halt the irremediable deterioration of Gol D. Roger's health. The latter was affected by an incurable disease, but Crocus prolonged his life, allowing him to maintain the strength required to reach Laugh Tale.
With the exception of Buggy and Shanks, the entire group of the Roger Pirates, including Crocus, visited Laugh Tale. As such, the medician is one of the first alive people to know the truth about what happened during the Void Century.
Following the disbandment of the Roger Pirates, Crocus returned to Twin Cape, where he now lives together with Laboon.
2) Jaguar D. Saul
One of the few people who have the rare trait of the initial D., Saul is a follower of Moral Justice and a former Marine Vice Admiral. After being ordered to lead a Buster Call against Ohara, whose inhabitants dared to research the Void Century, Saul reneged on the Marines.
As the Buster Call destroyed the island, Saul seemingly died while protecting Nico Robin, as fellow Vice Admiral Kuzan "Aokiji" left him frozen. In truth, Saul managed to survive, it is unclear whether by chance or due to Aokiji's intent.
Nevertheless, although covered in bandages, Saul led a group of giants to collect the books that the Ohara scholars had saved from destruction. He then transported the manuscripts to Elbaf and went into hiding there.
Saul likely read the books, gaining invaluable knowledge about the Void Century.
1) Kureha
With a stunning age of 141 years, Kureha is the oldest known human in the One Piece series. Despite being a doctor, she is frequently referred to as a witch for her eccentric behavior. She even threatens her patients if they do not follow her orders.
Despite her rough mannerism, Kureha is a good person. She took Chopper as a son, becoming his adoptive mother after the death of Dr Hiriluk. Interestingly, Kureha knew that the Pirate King was called Gol D. Roger, even if the World Government tried to conceal the initial D., changing the name to "Gold Roger."
Being so knowledgeable about the Will of D., which, as revealed in chapter 1085, is inherently tied to the Void Century, it's highly possible that Kureha is aware of the true history of the One Piece world.
Final Thoughts
While the secrets of the Void Century could be revealed to One Piece readers through one of the well-informed characters mentioned in the thread, they most likely won't be disclosed until the Straw Hats arrive in Laugh Tale, completing the same journey of the Roger Pirates, but with a different outcome.
Imu and the Five Elders are hiding the truth about the Void Century by any means necessary, as the disclosure of what truly happened during the war between the Twenty Kingdoms and the Ancient Alliance is the key to unmasking the tyrannical nature of the World Government.
Upon the discovery of the One Piece, Roger learned everything, which he later recounted to Edward Newgate. Shortly before his death in Marineford, Newgate declared that the World Government fears the day when someone will find the truth of history and carry its burden. This person will certainly be Monkey D. Luffy, now revealed to be the second coming of Joy Boy.