The "Will of D." is a mystery surrounding some people who have been put in tier D before championships
The initial was first pointed out in Oda's SBS questions and answers corner, when 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 the D in Luffy's name was brought up, as well as Oda's first hint toward a bigger mystery.
The "Will of D." has also been mentioned by Nico Robin three times, the first being when she rescued Luffy from quicksand during the Arabasta Arc; she asked Luffy about the meaning of the "D." and why those with the name fight, but he had no idea. The second was when Robin was talking to Gan Fall after they pulled up the ancient belfry. Gan Fall mentioned feeling that Luffy left the same impression on him as Gol D. Roger did. The third time was when Robin tried to ask Silvers Rayleigh about the significance of the "Will of D.", bringing up the Void Century at the same time. He did not reply on what the "D." stood for but stated that they (the Roger Pirates) had discovered the details of the Void Century; however, Robin declined his offer to tell her about it, determined to discover the truth herself.