So instead of posting headcanons and listing them as fact, I'm going to instead try and promote discussions about headcanons. Keep in mind, all these things I will be mentioning have not (I REPEAT) have not been confirmed in any way in the show or by Eiichiro Oda, but there is some evidence that leads us to believe it is possible or at least worth looking into. The idea is, I will present my findings in this card, and those of you that comment will discuss whether you think it's true. Make sense? Alright. And fair warning, there will be One Piece spoilers. On with it then.
Nico Robin, the archaeologist of the Straw Hat crew and sole survivor of the Ohara Massacre. She possesses the ability to read poneglyphs, something only a handful of named people have ever been able to do in history. She's considered to be the only living person who can do it (though this might not be completely true). They have information on the locations of Super Weapons that the World Government fears, though the schematics for Pluton was inscribed on a blue print and given to Cutty Flam, AKA Franky. You know... this guy.
They also contain information crucial to what is referred to as the Blank, or Void Century, the most important being the Rio Poneglyph. It contains the story of the history that has been forgotten. It is implied in the show, that whatever happened in this century was so important that anyone who discovers the events would be able to change this world. And Nico Robin is the only one who is capable of reading them.
So what are the qualifications of a D?
Beyond having D in their name (More on that later), there are really only two universal traits Ds share, though some traits are common among Ds, but not inherent amongst all of them.
The first is the ability to laugh in the face of death.
Ace, Roger, Saul, and even Luffy when he thought he was going to die at Lougetown. So what about Robin?
Yeah. While on Ohara, she met Jaguar D. Saul (a D) who taught her to laugh whenever she was crying, which she does when mourning his death. She also very casual talks about grim things like the slow and painful (or otherwise gruesome) death of her and her allies.
However... It's somewhat hard to say this is conclusive proof because Robin's never really been in a near death experience. Spandam almost dragged her away from her crew, it is unlikely she would have been killed. And beyond that, Robin is not a very expressive person by her nature. Even then, Marshal D. Teach (IE Blackbeard) did show a fear of death despite being a D, so even this is not 100% foolproof of a method.
That being said, I think we can move onto the second point.
The power to change the world, for good or evil.
As mentioned above, the Rio Poneglyph has the power to change the world, a power that is increased by Robin being in a crew with a D as a captain, who also happens to be the Future Pirate King (probably)...
The damning proof however, is the death of Jaguar D. Saul. Ds are supposed to have the power to change the world, but all he did was save Robin. This would seem to imply that she has more potential to affect that world than a confirmed D.
Saul does mention everyone in his family has the intial D, but none of them have been shown, which could imply that this is not the infamous Will of D.
However, his smiling in the face of death is in my mind enough evidence to dismiss this.
So then, if she is a D... why isn't she called Nico D. Robin?
The exact number of Ds is unknown, since many choose not to use the name D as it attracts unwanted attention.
Like remember Trafalgar D Law?
For one, Trafalgar Law just sounds cooler, but he intentionally left out the D to avoid scrutiny. Which begs the question...
Does Robin even know if that she is a D?
(Assuming she is one of course)
It is entirely possible that the fact that she was a D was kept from Robin by the Ohara Scholars. They might have known what the D means, which could possibly explain why the Marines didn't hesitate to destroy a place of knowledge. Their knowledge was dangerous.
Also, Robin doesn't even know who her father is, so we can be sure she was kept somewhat in the dark.
After all, is Aokiji new she was a D... would he have let her go?
Maybe the Ohara scholars saved her life that way.
So then... who is Nico Robin's father?
Her mother does not possess the will of D, and as far as we know, you cannot simply become a D (though that might be another card someday).
The most common theory is that her father is....
This is another headcanon and I might get into this later, but most people who believe Robin is a D think Dragon is her father. Yeah you heard me right, this would make her Luffy's older brother. But they do have similar features, though Robin gets most of them from her mother, who is for all intents and purposes identical to her.
The only difference is they have the same hair, while Olivia's was white.
He even meets up with her over the timeskip.
Olivia mentions that she needs to complete her husband's work, which could hint at Dragon's attempt to destroy the Government, something the history of the Blank Century very well could do.
Regardless of who Robin's father is, my question to you remains... does Nico Robin possess the Will of D?