Considering how famous the Straw Hat Pirate is, the crew that serve under its banner is relatively small. Excluding Luffy, there are currently nine people who serve as the crew of the Straw Hat Pirates. In comparison, fellow Supernovas like the Fire Tanks Pirates and Hawkins Pirates have way more crew members than the Straw Hats.
Needless to say, with such a skeletal crew, every member has a highly specific role to fill. Zoro as the main combatant, Nami as the navigator, Sanji as the chef, Chopper as the doctor, Usopp as the gunner, Franky as the shipwright, Brook as the musician, and also Jinbe as the helmsman. But what about Nico Robin? What is the role of an archeologist like her in the story?
Who is Nico Robin?
To better understand Nico Robin's role in the story, let's briefly talk about who she is first. The Straw Hat crew's first meet Robin in Arabasta. She was the vice president of Crocodile's Baroque Works, codenamed "Miss All Sunday". She joined the Straw Hat Pirates at the end of the Arabasta arc.
Robin is the sole survivor of Ohara island, a little country in West Blue where most of the world-renowned archeologists and historians live and study. The Ohara island was annihilated by the Buster Call because the world government deemed their knowledge regarding the void century as a threat to them.
As the sole survivor of the Ohara people and the only person in the world who has the ability to read the Poneglyphs, Nico Robin is number one on the world government's most wanted list. They even went as far as labeling her as the Demon Child in order to get regular people to despise Robin.
However, Robin's thirst for knowledge is still alive and strong. As a matter of fact, the main reason why Robin wanted to join the Straw Hats at the time was in order to explore the world, find the Poneglyphs, and uncover the secrets of the Void Century.
What is Robin's Role in the Crew?
As mentioned before, everybody in the Straw Hat Pirates has a specific role to fill, so what is the role of an archeologist like Robin in the day-to-day activity of the Straw Hats? Well, she's responsible for quite a lot of things, actually.
One of the most important duties of Nico Robin is as the source of knowledge for her crew mate. Robin is without a doubt the most knowledgeable person within the Straw Hat crew. As a historian, she has a vast knowledge about all sorts of things. Such as the time when Robin told Nami to follow the Log Pose which points the way to the Skypiea, and deduce the true identity of the crew of St. Briss based on the skull that she just reconstructed.
Not to mention the years she spent running and hiding as a fugitive from the world government and her time working in the underworld with Crocodile also enabled her to garner so much information that most government officials won't even know about. Such as information about the Revolutionary Army, the Cipher Pols, and also the Germa 66.
What is Robin's Role in the Story?
One Piece has three storylines going on at the same time. The first one is the story about whatever story arc that they are currently going through. The second one is the grand adventure of Luffy and his dream to find One Piece and be the Pirate King. The last one is the investigation of the secrets of the world, such as the Void Century, the Ancient Weapons, and the legendary Joy Boy. The other crew members should be sufficient to finish the first storyline, but Robin is crucial for the second and the third ones. After all, only those who can read the Poneglyphs can point the direction to Laugh Tale, the last island where Gol D. Roger left the One Piece, along with other mountains of treasures.
As for the third storyline, Robin acts as a way for Eiichiro Oda to tell the deeper and grander lore within the world of One Piece. By letting her become some sort of detective that puts the pieces together through the clues that they found during their journey, Oda has found a subtle world building technique that will keep the readers interested in the world outside the Straw Hat Pirates. Without the third storyline, all we get is simply a bizarre and exciting adventure of Luffy and his friends. Nobody would be interested to know more about the world in which they live.
However, by letting us know about the ancient weapon called Poseidon through the Poneglyph in Skypiea and the Joy Boy’s apology letter in the Fish Man island's Poneglyph, and other relevant information from the rest of the Poneglyphs, the readers are now interested to know what happened to the world before the time of the Straw Hats. Not only that, it also connects the entire places that they've been through in one continuous narrative. And that is ultimately what the role of Nico Robin is to the story of One Piece.