Will of D.

In the One Piece fictional universe, few characters have the middle initial "D". This mysterious middle initial has been called "the Will of the D" (Dの一族, D No Ichizoku) or "D's will" by several characters in One Piece. Some translations refer to it as "Gol D's will" in reference to Gol D. Roger, the late pirate king. The English dub by 4Kids chooses to refer to the D as the "the Spirit of Gol. D". In the recap supplement Grand Line Times, they were referred to those who carry it as "The men of D."

What is the D?
Inherited will, the swelling of the changing times, And the dreams of people. These are things that cannot be stopped. As long as people seek the answer to freedom, these will never stop!

One Piece, second intro and words heard in Loguetown.

It is not clear what the significance of the shared middle initial is, but it appears that all share a similar faith in their own (and others') dreams and/or destiny. Furthermore, they seem to be able to withstand incredible punishment in battle and only surrender to death when they know there truly is no way of avoiding it.

In Oda's SBS questions and answers corner, 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 Luffy's 'D' was brought up, as well as Oda's first hint towards a bigger mystery.

In the storyline, Dr. Kureha is the first to mention The Will Of D and it was revealed by her that Gold Roger's (the late pirate king) true name was Gol D. Roger. She tells Dalton that Chopper has joined up with a very dangerous man.

So it still lives... The will of D.

- Dr. Kureha speaks to Dalton about Luffy

Recently, Robin tried to ask Silvers Rayleigh about the significance of The Will Of D at the same time as mentioning the Void Century. While he did not reply on what the D stood for, Silvers stated that they (The Jolly Roger Pirates) discovered the details on the Void Century, but when he offers to tell her, she turns down his information. Robin agrees to discover the truth herself.

Family
One clue about those who carry the "D" was left by Saul who specifically said that while he didn't know the meaning of the initial, he did know everyone in his family carried it in their names. .

So far, its seems that the Ds all may indeed be related to each other. Luffy does have a grandfather, Vice-Admiral Garp, whose full name is Monkey D. Garp. Luffy is also the son of the revolutionary Monkey D. Dragon. In addition, Luffy's brother, Portgas D. Ace, has also been revealed to be blood related to Monkey D. Dragon and Monkey D. Garp as well.

Known Family Names

 * Monkey
 * Gol
 * Marshall
 * Jaguar

The Concept of Death
An odd trait of the D's is related to death. Often they are seen laughing or smiling greatly when they are about to die, as if they seem to accept their death when they cannot escape it and display no apparent fear toward death. The reactions of many characters who witness this is often shock or disbelief because it is hard to imagine anyone not being afraid of dying at all. So far this strange habit has been shown 3 times:
 * Witnesses claim Gol D. Roger died smiling during his execution.
 * 22 years later one of those witnesses, Captain Smoker, witnessed Luffy smiling in the same manner as Gol D. Roger when Buggy was about to kill him.
 * Nico Robin also witnessed Saul laughing and smiling as he was frozen by Aokiji.

There was almost a fourth instance which was the duel between Ace and Blackbeard, in which Ace smiles before his final attack against Blackbeard. He did not die and was only defeated and sent to Impel Down but probably thought he would either win or die. Blackbeard was also smiling and even laughing.

Their Knowledge on Themselves
The biggest mystery with the D's is that those that carry the D. themselves do not know fully what it means either. When asked by Nico Robin why those that carry the D. fight, Monkey D. Luffy was completely clueless about what she was talking about. Jaguar D. Saul also displayed an absence of knowledge on the subject of the D when she asked him what it meant.

Alive
At 17 Luffy is the youngest pictured D. carrier thus far in the story. Monkey D. Garp appears to be the oldest known living carrier of the D. Thus far in the story, there have been no female D's.
 * Monkey D. Luffy (Known as "Straw Hat Luffy"): Captain of the Straw Hat Pirates
 * Portgas D. Ace (Formerly Monkey D. Ace, known as "Fire Fist Ace"): Commander of the second division of the Whitebeard Pirates
 * Monkey D. Dragon (Known as "Revolutionary Dragon"): Leader of the Revolutionaries
 * Monkey D. Garp (Known as "Garp the Fist"): Marine Vice Admiral
 * Marshall D. Teach (Known as "Blackbeard"): Former subordinate of the Whitebeard Pirates second division, Captain of the Blackbeard Pirates, Shichibukai member

Deceased
In the present storyline, Roger has been dead for 22 years. Saul has been dead for twenty years. Saul was a giant, making him the only apparent non-human "man of D" in the storyline. As a giant, his age may have been anywhere up to 300 years (the normal lifespan of a giant).
 * Gol D. Roger (Known as "Pirate King"): Captain of the Jolly Roger Pirates; Turned himself in for execution after being diagnosed with a fatal illness.
 * Jaguar D. Saul: Former Marine Vice Admiral; Killed by Vice Admiral Kuzan for betraying the Marines.