Nearly everyone with the will of D is defiant towards the World Government. They are also defiant towards death. Nefertari D. Lili defied the World Government.
D could also stand for Democracy if you consider the destruction of the great kingdom. The World Government considered the Great Kingdom to be a dangerous enemy, which could be because they had opposing political beliefs. Nefertari D. Lili could have betrayed them to try and prevent a global autocracy.