One Piece is a Japanese manga written and illustrated by Eiichiro Oda. This manga is basically about adventure and comedy. It's about a boy Monkey D. Luffy, a seven-year-old boy, who ate a devil fruit, because of which his body gained rubber-like properties.
This manga series has various important characters, one of them is Joy Boy. however, we’ve yet to be introduced to him. According to the story so far, we know that Joy Boy is a prominent Figure during the Turbulent Era, like some 800 years before the latest storyline of the manga series.
In spite of the fact that most of the information about this man hasn't been known yet, there are a few hidden details that Eiichiro Oda has slipped into the story that we feel are worth checking out.
There are many theories about Joy Boy that would definitely help us to know who he really is. What do you think the theories are going to be? Will the theories about him tell us who he really was or are they just assumptions by people?
Who is Joy Boy?

Joy Boy was an extremely relevant figure of the Void Century. He was first mentioned at Fishman Island, where Nico Robin found his message in the Sea Forest. To be more accurate he was first mentioned in One Piece Chapter 628. In the manga series, Joy Boy made his introduction a bit earlier, in episode 548.
Joy Boy is known to have some connections to Fishman Island. He wanted the Fishman to live with the humans on the surface, but for some reason, he failed to keep his promise.
Because of this he was supposed to write an apology and pass it down to those who inhabited the island. Also, he was mentioned by Gol D Roger in the manga series. Roger said that he wished the two could have lived during the same period upon learning the true history of the Laugh tale.
What is the Theory behind Joy Boy?

The very first theory behind Joy Boy is that his Will is the Will of “D”.
He is said to be a man from the void century dating back to at least 800 years ago, he must have been linked with some Ancient kingdom. The fact that his phone glyph was found at Fishman island solidifies this fact.
Moreover, it is theorized that he was not only from the ancient kingdom, but he was also the king and a member of the D clan.
In addition to his origin, his actions are what garners the most attention. Before the destruction of the kingdom, his last action was to relay his apologies to the mermaid princess for failing to fulfill his promise.
Despite his failure, he left this duty to someone else.
To make this theory more clear we can say that here Luffy’s promise is Joy Boy’s promise, which means to take Poseidon aka Shirahoshi, and the Fishman to an outside world to stand equal.
Also after the first mention of him in the series, it was largely assumed that it was the name of a person that existed during the Void Century, as mentioned above which might be true.
Moreover, this is a theory that seems to be gaining a lot of support. There have been many more theories about Joy Boy in the manga series, which might be true. But what according to you would be a suitable theory for Joy Boy?
Thanks for reading!