Will of the D. (One Piece)

In the series, few characters have the middle initial "D.". This mysterious middle initial has been called "the Will of D" (Dの意志, D no Ishi) or "D's will" by several characters in One Piece. Some translations mistakenly 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 "Gol D.'s will", while the FUNimation dub refers to it simply as "the Spirit of D." both deriving from a mistranslation of the original Japanese text. In the recap supplement Grand Line Times, they were referred to as "The men of D."

All introduced bearers of the initial D

The Mystery of D.

The bloodline of the Gol family.

The initial was first pointed out in Oda's SBS questions and answers corner, he was also 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, Nico Robin is the first to mention The Will of D. and it was revealed by Kureha 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.

”

— Kureha speaks to Dalton about Luffy.

The Will of D. is also talked about by Robin three times, the first was during the Alabasta arc as she asked Luffy about the meaning of 'D' although Luffy had no idea. The second was when Robin was talking to Gan Fall after they pulled up the ancient belfry. Gan Fall talks of Luffy making the same impression that Gol D. Roger did on him. He says to Robin, "It suits him to have a name of just one letter". The third time was when Robin tried to ask Silvers Rayleigh about the significance of The Will of the D. and at the same time mentioning the Void Century. While he did not reply on what the D stood for, Rayleigh stated that they (The Roger Pirates) discovered the details of the Void Century, but when he offers to tell her, she turns down his offer. Robin agreed to discover the truth herself. 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.

So far everyone in the Monkey Family has the initial "D".

So far, its seems that the Ds all may indeed be related to each other, if not by blood then at least by their fates. 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 (Who took his mother’s maiden name in rejection of his father), Portgas D. Ace, has also been revealed to be blood related to Gol D. Roger and Portgas D. Rouge. Whitebeard noted that killing the bloodline of a D does not put out the flames that they (a person carrying the D) carry and someone else simply will pick it up one day. Later on Trafalgar Law states "What's the Dark King thinking this time? The Will of the D. will almost certainly call up a storm again." It is still not stated why he said that, 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 Gorosei stated that "D" means danger.

"Inherited Will"

“

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 cease to be!

”

— One Piece, second intro and words heard in Loguetown when Monkey D. Dragon appears

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. Whenever a D. appears, a series of massive events can occur in the One Piece world particularly when they get involved directly, this has included the start of a new era, a war and many change of governments.
The biggest mystery with the D's is that most of those who carry the D. themselves do not know fully what it means either. Jaguar D. Saul displayed an absence of knowledge on the subject of the D. when Nico Robin asked him what it meant. When she asked Monkey D. Luffy why those that carry the D. fight, he was also completely clueless as to what she was talking about. Though it seems that the Marines are somewhat aware of the D's as Sengoku showed interest in Ace because of it.
Gol D. Roger also displayed a knowledge of it in a dialogue between him and Whitebeard. He begins "Long ago far far away" but it is cutoff before he can finish. He is so far the first and only one known by the storyline to have discovered what it means as a D. himself, although other D.s may also know it but none are confirmed.

Ace dies with a smile on his face; a common 'trait'
seen on every "D." near or at their deaths.

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. An odd trait of the D's is related to this. 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 5 times:

• Witnesses claim Gol D. Roger died smiling during his execution.
• 22 years later one of those witnesses, Smoker, also 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 to death.
• Portgas D. Rouge is seen smiling during the naming of Ace right before her death as a result of the childbirth.
• Portgas D. Ace was smiling after he thanked everyone for loving him right before his death.

There was almost another instance which was the duel between Ace and Blackbeard, in which both Ace and Blackbeard smile before their final attack against each other. Though Blackbeard did show a strong belief in his devil fruit and did probably not think he would die, and he did not. Ace did not die and was only defeated and sent to Impel Down, but both may have thought they would either win or die.
Most notable, the mention of "will" creeps up on numerous occasions; Whitebeard himself went on to explain that when they die, another may pick up their "will" at a later date and carrying it on their back and fight for it, even if the bloodline dies. Before Whitebeard's death, it is revealed through a flashback that he was told by Gol D. Roger of the true meaning behind the Will of D; he claims that Roger is waiting for a certain man with the initial D. He noted that Teach is not the one who is carrying "Roger's Will" which also includes "Ace's will" and that one day it will be picked up by another.
However, Marshall D. Teach did show a fear of death, an odd trait amongst the men of D. When Whitebeard attacked him at Marineford, Blackbeard started to panic and became fearful saying "You would not do this to your son, would you?" Blackbeard, even though he did not fear it when facing Ace, did not accept his death when it was before him. He did not die from that incident due to his own strength, but he still feared it nonetheless. He is the first D. to show this fear when faced with the possibility of death.