Nefertari, Vivi

ネフェルタリ・ビビ

Nefertari Vivi first appears as an antagonist when the main characters reach the Grand Line, serving as a member of Baroque Works, a criminal organization responsible for several scandals in the country of Alabasta. It is revealed that Vivi is actually the princess of Alabasta, but she infiltrated Baroque Works as Miss Wednesday to uncover the identity of the leader, who turns out to be the Shichibukai member Sir Crocodile. With help from the Straw Hat Crew, she was able to reveal the truth to her people, ending several uprisings and stopping battles that would have forever ruined the nation.
Trivia
The word "Nefertari" in Vivi and her father's names is ancient Egyptian, and means "the most beautiful" or "most wonderful". It was a word often used to describe Egyptian queens and princesses. Oda admitted the Alabasta arc was based around ancient Egypt, but apparently had no clue on what the word meant when he chose the name.
Despite her long absence from the series and her separation from ties to the Straw Hat crew, Vivi was featured on the cover page of chapters 271, 423 and 484, usually reserved to Straw Hats. That makes her the only non-Straw Hat other than the Going Merry to appear on a cover page.
In the English manga and anime, her name is romanized as "Nefeltari Vivi".