Emperor

The title of Emperor (feminine Empress ) was held by certain officials and heads of state in certain governments. It was usually inherited, but this was not always the case. Sometimes successful warlords and conquerors would create Empires by declaring themselves emperor. It is uncertain how, or even whether, an Emperor's position and authority differed from that of a King.

Since Oda has not introduce an Emperors in his story yet but he did introduce 2 Kuja Empress name Boa Hancock, even thought she was an Pirate and a Shichibukai, she was in fact a real ruler of the island. Elder Nyon however was the former Empress who left Amazon Lily, Hancock replaced her and pardon her when she returned.

There may be other Emperors/Empress not yet in the story to date

Pirate Emperors
However the Yonkous may rule the New World, however they are not a true royal Emporers.