Aka Robin Nico Aka Miss All Sunday.

Robin spent her early youth looking for ways to construct WMDs together with some renegade scientists. When the world government found out, it managed to avoid the worst. However since Robin was still a minor she wasn't incarcerated. She started to seek out the worlds worst criminals for support in her research, betrayed them one after another and left a wake of destruction behind her.
She was most prominent as the right hand of Sir Crocodile, a rogue turned vassal of the world government who built an underground army and secretly sought to overthrow the country of Alabastia.
After Crocodile had no more information to offer to her she finally joined the Strawhats upon personal request rather than like everyone else being invited.
Her research has since made huge leaps forward.

After her involvement in the destruction of the government of Dressrosa as part of the Strawhat Crew her bounty was raised to 130 million berry, making hers the highest bounty after the monster trio consisting of Luffy, Zorro and Sanji.

https://www.youtube.com/watch?v=QhXoLhl-C6I