The Will of D

What is the D?
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. Further more, 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 answer 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.

Dr. Kureha: So it still lives... The will of D.

Personality
One of the most common reference made about the Ds are the constant remarks on how similar they are to each other. Although their overall personalities vary between each D, they are typically simple-minded people who see the simple solutions to things, however do not always act stupid as a result of it. They display fun-loving traits and are able to make friends or get along with most people without there seeming to be any cause for alarm. They are also highly competitive (even with each other) often aiming their ambitions at high targets, but unlike normal people seem to have the determination, strength and will power to carry out their ambitions.

Nico Robin states, she has met a number of the Ds over the years and all the people she's met with D as their middle initial are incredibly strong and equally just as stubborn. Other references come from various characters:
 * Gan Fall who seemed to have known Gol D. Roger and found similarities between Luffy and the former pirate king.
 * Admiral Aokiji equally noted the similiarity between Luffy and his Grandfather, although he wasn't sure if it was stupidity they shared in common or simple mindedness.
 * Sea Train engineer Kokoro mentioned that if the legendary shipwright Tom, who built Gol D Roger's ship, were still alive, he would have helped Luffy with the raid on Enies Lobby because of the similarities between Luffy and Roger's personalities.

Initally, thus far none of the Ds have done anything truly worthy of being called "evil" despite everything that has happened in the storyline. For the most part, they don't seem to take sides with anyone, appearing "neutral" until a situation calls into effect their strong sense of "personal justice". This individualistic trait makes them drawn to the pirate lifestyle, as in all the D's thus far have been powerful and famous pirates except Jaguar D. Saulo, who abandoned and became wanted by the Marines because he pursued his own personal justice.

Family
One clue about those who carry the "D" was left by Saulo 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, this is supported by Luffy and his brother Ace who are both carriers of the D and share no other part of their name with each other. . One of Luffy's grandfathers was revealed to be Vice Admiral Garp recently, but it remains a mystery whether or not he carries the D. However, despite this evidence, this does not thus far rule out the intial existing in other family lines as well.

Since family may also be regarded as the status of close group of people (especially in the world of One Piece) the Ds may not be directly related to each other at all, but rather a group individuals who have come to accept each other as if they were a close family.

Fate
Another odd thing about the Ds is that they appear to encourage or ignite change whenever they appear somewhere, sometimes it seems as if they are drawn to fate that is playing out. For example, Blackbeard's raid is the turning point for the Drum Kingdom and their selfish and corrupt King was made to flee the kingdom freeing the citizens from his corrupt reign. Later when he returns to Drum Kingdom, Luffy's intervention stops Warpol from taking his throne back. Jaguar D. Saulo also ended up on Ohara island before the Buster call was due to arrive and was there when it came to help Robin escape, thus allowing the only reader of the ancient langange to continue to live. Robin states she is convinced Luffy will play an important role in the events of the world after seeing a note left by Gol D. Roger in Skypiea near its sacred bell.

How fate reacts to the Ds cannot be fully explained but an example of fate openingly effecting a D comes from Blackbeard after leaving a bar in Mocktown on the island of Jaya. Blackbeard encountered Bellamy who was entering at that moment, while they did not say anything to each other, Blackbeard seemed to sense something was about to happen and hung around outside.

Dreams
They seem also drawn to dreams, for example the places Luffy visits usually result in him ending the dreams of the many One Piece villains, including Captain Kuro, Arlong and Sir Crocodile. In the outcome of every battle, he leaves them alive to see their failed dreams fall apart, in the process, the victims of these villains have their dreams and hopes renewed. Every member of Luffy's crew also have a dream they are trying to succeed with. Blackbeard also encourage Luffy to ignore Bellamy's talks about the end of the dreams of pirates, giving Luffy a speech about how a Pirate's dream never ends.

The concept of death
A odd trait of the Ds 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 Gold Roger died smiling during his execution.
 * 22 years later one of those witnesses, Captain Smoker, witnessed Luffy smiling in the same manner as Gold Roger when Buggy was about to kill him.
 * Nico Robin also witnessed Saulo laughing and smiling as he was frozen by Aokiji.

Their knowledge on themselves
The biggest mystery with the Ds is it seems 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. Saulo also displayed an absence of knowledge on the subject of the D when as a child she asked him what it meant.

Theories
During the course of the series, since the D was first brought up, many fans have come up with theories on what the D means. Most theories have been unproven thus far into the storyline as most of these are based on minute pieces of information that lack any real proof or hardcore evidence. While Oda has ensured the fandom the truth will be revealed in time, the more common and/or acceptable theories are:
 * 1) The Ds are related to each other - This was an old theory based on Luffy and his brother Ace.  This since has been proven as true since Jaguar D. Saulo stated that he knew all those carrying it were part of his family, however the exact meaning of family is sometimes called into question as 'family' in One Piece has more than one meaning.  For example, the Straw Hats are considered a family even though none of them are related by blood.
 * 2) The Ds are the descendants of the ancient race - another theory is that they are the descendants of the ancient race that died out some 800 years ago during the 'Blank Century' mentioned in the poneglyphs.  The main suggestion to this theory is based on Professor Clover not being allowed to say what the name of the lost civilisation was during a speech on Ohara Island.  Some theories point to the 'D' being the first letter of this race. Also, somehow Gold Roger had learnt how to read the Poneglyphs, and his knowledge even included how to write in the language.

Alive
At 17 Luffy is the youngest pictured D. carrier thus far in the story and Luffy's brother Ace the second youngest at 20 years old. Marshall D. Teach's age has not been revealed but he appears to be the oldest known living carrier of the D. But currently, Vice Admiral Garp could be the oldest according to the latest chapters. Thus far in the story, there have been no female Ds.

Deceased
In the present storyline, Roger has been dead for 22 years. Saulo (who is presumed dead) has been dead for twenty years. Saulo was a giant, making him the only apparent non-human D. (bringing into question how a giant ended up as a D.) carrying the D in the storyline. As a giant, his age may have been anywhere up to 300 years (the normal lifespan of a giant).