Duval was once the thuggish leader of a kidnapping organization, but he is now the leader of the Rosy Life Riders, formerly known around Sabaody Archipelago as the Flying Fish Riders. His face was drastically altered by Sanji by a violent series of kicks and now finds himself extremely handsome.
Duval is a remarkably goofy and arrogant man. He is also rather large in size. He was once a bandit, but due to his appearance being so remarkably close to the Wanted Poster of Sanji. He had to run from the authorities. After he crossed paths with Sanji. His face was completely altered dramatically. It becomes a comedic gag that he can no longer wink properly. His face will contort awkwardly.
After his alteration, his arrogance only just increased. He also became quite loyal in gratitude to Sanji. He was of great assistance to the Straw Hat crew when they needed to find the kidnappers of the the mermaid Camie. Since Duval once worked as a professional kidnapper. He knew there to look and who might of had a role in it.
Duval was originally created by Eiichiro Oda for use in the One Piece manga, and it is not fully known as to what inspired his creation. Duval first appears both in One Piece: Volume 50 and One Piece #386.
Duval was born short after the execution of the Pirate King, Gold Roger. His face was so awkward it was thought to be the curse of the pirate king. He grew up to be a thug in a quiet town, but events of his life would be shattered do due a drastic twist of fate. It was after the Straw Hat Pirates had assaulted the World Government base of Enies Lobby. One of the Straw Hats, Sanji, was the only one who didn't have a photo for his wanted poster. Instead a sketch was used. Though this sketch didn't look very much like Sanji. It bore a remarkable resemblance to Duval.
Once this poster had spread far enough. Duval was now on the run from the Marines, who before payed little attention to such a small fry. They had mistaken him for the wanted man, Sanji.
Duval was so embittered by the loss of his quiet crime-filled life. He swore revenge against The Black-Leg Sanji To hide his face from further persecution. He took to wearing an iron mask. This earned him the nickname, Iron Mask Duval. He teamed up with others to create a kidnapping gang and waited near the end of the first half of the Grand Line, where Duval knew all pirates would be heading. This gang was called the Flying Fish Riders. They would kidnap pirates or anyone else who crossed their way and sell them to the slave market auction houses at Sabaody Archipelago. This was all just to wait for Sanji to appear.
Eventually, Duval's goal became true. He was already working with some mer-folks to kidnap the mermaid Camie. They were using Hatchan for bait. To Duvals's great surprise, his men reported that they had spotted Camie, and she was with the Straw Hat Pirates.
Duval stayed hidden for most of the battle with the Straw Hats. That was until Luffy came crashing through his roof and got a look at his face without the iron mask. He tried to attack Luffy and the battle broke out into the open. It was Luffy that exposed Duval's face by knocking off his mask. Everyone was astounded when they saw just how close he resembled Sanji''s wanted poster. Sanji though was absolutely enraged. He was not even slightly happen about his wanted poster. Hearing Duval''s sob story about the trouble it caused him only made Sanji more angry. He dove into the water and came to Duval. They battled and Sanji so severely kicked Duval with his feet. That he purposely changed his bone structure to give him a totally different face.
Duval reappeared to the crew as they were enjoying Hatchan's takoyaki. He was a much happier person, but his personality wasn't too different. He was just more arrogant and spent too much time looking at himself in a hand mirror.
Out of gratitude, Duval gave Sanji his Den Den Mushi number in case he ever needed assistance. This number would actually come in very handy.