2. Bartholomew Kuma
Bartholomew Kuma was one of the most important members of the Revolutionary army. He infiltrated the World Government by becoming a fearsome pirate, and hence became a Shichibukai.
The Reason for Kuma to infiltrate the World Government is still unknown. But, it was mentioned by Doflamingo that he became a slave of the World Government and also made a sort of a deal with them.