The initial was first pointed out in Oda's SBS questions and answers corner, when he was asked what the D. in Luffy's name stood for. Oda just replied to read it as a D. for now and that he would reveal the truth behind it in time. This was the first time the D in Luffy's name was brought up, as well as Oda's first hint toward a bigger mystery.
Kureha was the first to mention the "will of D." when she revealed that the true name of the late Pirate King was Gol D. Roger. She told Dalton that Chopper had joined up with a very dangerous man.
The "Will of D." has also been mentioned by Nico Robin three times, the first being when she rescued Luffy from quicksand during the Arabasta Arc; she asked Luffy about the meaning of the "D." and why those with the name fight, but he had no idea. The second was when Robin was talking to Gan Fall after they pulled up the ancient belfry. Gan Fall mentioned feeling that Luffy left the same impression on him as Gol D. Roger did. The third time was when Robin tried to ask Silvers Rayleigh about the significance of the "Will of D.", bringing up the Void Century at the same time. He did not reply on what the "D." stood for but stated that they (the Roger Pirates) had discovered the details of the Void Century; however, Robin declined his offer to tell her about it, determined to discover the truth herself.
In contrast to the openness of Luffy and other "D"-bearers, there are some "D." families who seem to know its meaning and try to keep it secret from the public, such as Trafalgar Law's.
One clue about those who carry the "D." was left by Jaguar D. Saul, who specifically said that, while he did not know the meaning of the initial, he did know everyone in his family carried it in their names. This implies that the "D." is passed down between generations. So far, it seems that all characters with the "D." may indeed be related to each other, if not by blood, then at least by fate. An example of the hereditary nature of "D." can be seen in the Monkey family, consisting of Monkey D. Luffy, his biological father Monkey D. Dragon, and his grandfather Monkey D. Garp. This, however, does not necessarily imply that all "D."-bearers are closely related; some of the "D."-bearers have had children together, as seen with Portgas D. Rouge and Gol D. Roger, and some of the "D."-bearers are giants.
An apparently unique event within the enigma of "D." was the birth of Portgas D. Ace, whose parents, Portgas D. Rouge and Gol D. Roger, both bore the mysterious initial.
During a flashback, Gol D. Roger complains that the World Government is calling him "Gold Roger" instead of his real name. He was then asked by Whitebeard what "D" is. Roger cryptically agreed to explain it, though the scene fades before he actually says anything. However, it is implied that Whitebeard has a clear understanding to its meaning. He later noted during the battle at Marineford that killing someone with a "D." would not extinguish the flames of their will because someone would eventually inherit that will and carry on Roger's wish.
Later on, Trafalgar Law wondered aloud that the Will of D. will almost certainly call up a storm again. It is still not clear exactly what he meant, but it hints that Silvers Rayleigh had a clear purpose for wanting to find Monkey D. Luffy on Amazon Lily. In the Post-War arc, one of the Five Elders stated that "D." means danger. After the timeskip, Law mentioned the Will of D. once more when Doflamingo asked him why he had so much faith in Luffy.
In certain places, the clan of D. have been called by another name, God's Archenemy.
— Rosinante telling Law about the D.'s true purpose.
When Donquixote Rosinante learned that Law is a "D.", he revealed to Law that those with this initial in their name are from the fated family of D. (Dの一族,Dī no Ichizoku?, VIZ: People of the D). In the country where Rosinante came from, children were told that if they misbehaved, they would be eaten up by "D.". Similarly, elders of that country seemed quite wary of the "storm" that those with "D." were known to cause. Rosinante explained to Law that the "D." has been secretly inherited worldwide and throughout history, and that in a certain land, the "family of D." was referred to as Natural Enemy of God (神の天敵,Kami no Tenteki?). According to Rosinante, the "D."s are considered to be the natural enemy of the Celestial Dragons, as Celestial Dragons are considered to be "Gods".
It is not clear what the significance of the shared initial is, but it appears that all share a deep desire for freedom and have strong faith in their own (and others') dreams and/or destiny. When a "D." appears, the general consensus is that they are going to stir things up on a global scale, for better or worse. Examples of world-shaking events instigated by "D."s include wars, changes in government, and the dawn of a new era. The Will of D. could be seen as an example of the broader concept of Inherited Will.
The biggest mystery of "D." is that most of those who carry the "D." are themselves unaware of its true meaning. Jaguar D. Saul displayed a lack of knowledge on the subject when Nico Robin asked him what it meant. Similarly, when she asked Monkey D. Luffy why those that carry the "D." fight, he was completely clueless as to what she was talking about. However, it seems that the Marines have some awareness of the initial's significance, as Sengoku showed interest in Ace because of it. When Sengoku is explicitly asked by Law if he knows about the meaning of the "D.", he gives a vague answer, but he seems shocked to find out that Law also carries the initial and he thinks about the checkered fate of the "D." carriers.
Trafalgar Law's parents also seemed to have some knowledge about what "D." meant, considering that although they passed down the initial to their child, they hid it and insisted on using only their surname and given names. Gol D. Roger is the first and, as of yet, only known "D."-bearer in the storyline to have discovered its meaning.
Those who bear the "D." in their name seem to be able to withstand an incredible degree of punishment in battle and only surrender to death when they know it is truly unavoidable. In the same vein is an unusual behavior that seems to be common to "D." – they are often seen laughing or smiling right before they die (or think they're about to die), as though they have accepted their fate and hold no fear of death. Observers have expressed shock and confusion at this phenomenon, unable to believe that someone could so thoroughly come to terms with the end of their life.
So far this strange trait has been shown 6 times:
Witnesses claim Gol D. Roger died smiling during his execution.
Smoker saw Luffy smile in the same way when he was about to be killed by Buggy, though he survived in the end.
Nico Robin witnessed Saul laughing and smiling as he was frozen to death.
Portgas D. Rouge is seen both crying and smiling when she named Ace, dying shortly after.
Portgas D. Ace thanked everyone for loving him and died with a smile on his face.
Trafalgar D. Water Law smirked at, mocked, and flipped off Donquixote Doflamingo when the latter was pointing a gun at his heart and preparing to shoot him, though he survived in the end.
Blackbeard is unusual among those with the "D." in that he has shown fear of death. When Whitebeard attacked him at Marineford, he started to panic and begged for mercy. Blackbeard, though he did not show fear when fighting Ace, could not accept death when it became a strong possibility.
Other traits some D. bearers have shown to be sharing include:
Narcolepsy: Witnessed in Luffy, his grandfather Garp, and Ace (later revealed not to be their biological relation). These people have been shown to fall asleep at random times, even in battle.
Appetite: Ds like Teach, Luffy, Garp, and Ace have been shown to consume enormous amounts of food in one sitting.
Spirit and charisma: Many D. bearers have a strong sense of purpose which naturally draws people to them.
Translation and Dub Issues
The 4Kids dub chose to refer to the Will of D. as "Gol D.'s spirit", in reference to Gol D. Roger, while the Funimation dub refers to it as the "spirit of D" in the Drum Island Arc. Both of these derive from a mistranslation of the original Japanese text. 4Kids' use of "Gol D.'s spirit" in Dr. Kureha's conversation with Dalton can be considered erroneous. While Gol D. Roger is spoken of in previous lines, Kureha made no mention of Roger in that particular line of text in the original Japanese.
In the recap supplement Grand Times, the holders of the D. were referred to as the "Men of D."
When Whitebeard was discussing the Will of D., he mentioned One Piece in the same statement, implying that "D." and the One Piece are connected by more than just Roger. This was not the only time that both were mentioned in the same chapter: Usopp and Robin asked Rayleigh about One Piece, the Void Century, and the Will of D., all within one conversation.
Although the term "Natural Enemy of God" seems to specifically refer to the Celestial Dragons as the god, Monkey D. Luffy also fought and defeated Enel, who held the title of "God" of Skypiea. During their fight, Nami referred to Luffy as Enel's "natural enemy", though this was due to Luffy's rubber powers canceling out Enel's lightning powers.
Trafalgar Lami is the only known named blood relative to a D. carrier who isn't a confirmed D. carrier as well.
↑SBSOne Piece Manga — Vol. 8, A fan asked about the D. in Luffy's name.