Shichibukai

The Shichibukai, the full title being Oka Shichibukai (王下七武海), are seven powerful and notorious pirates who have allied themselves with the World Government.

Overview
The Shichibukai is an organization of seven pirates turned privateers in the service of the World Government. Although they work for the World Government, the Shichibukai themselves typically do not care about nor respect the Government or even other Shichibukai, and are often considered by the Marines to be no different from any other pirate. On the other hand, they are known as "government dogs" and are despised by other pirates, but are still feared and respected for their infamous reputation and strength which they gained before, and even after they become Shichibukai.

The Shichibukai seem to be a mixture of the more chaotic Yonko and the more orderly Marines. The majority of the Shichibukai are composed of New World veterans; those who have met the Yonko or even fought against them.

The terms that the Shichibukai are on with each other is best described as very unstable. They have very little (if any) respect for each other and will for the most part not accept help from one another, apparently due to a firm sense of pride.

Becoming a Shichibukai
A pirate may choose to join the Shichibukai if a space becomes vacant (which means that at least one Shichibukai must resign or be expelled from the group, the former being Jinbe's case and the latter being Crocodile's case). However, in order for one to become a Shichibukai once this position opens up, one must be able to exhibit their strength against other pirates

Duties of the Shichibukai
They do not express open defiance to the authority of the World Government, but are not obligated to follow its orders either. They are technically only allowed to attack other pirates, and must also compensate the Government with a portion of their plunder, although this regulation is not followed most of the time. Each member of the Shichibukai must also have strong intimidation skills and a fearsome image to put off others becoming a pirate and make them fear them. In addition, given the discussion between Bartholomew Kuma and the Gorosei, it can be assumed that once someone becomes appointed to the Shichibukai they must be undefeated to maintain their position, or at least are not allowed to have a widely known debacle or scandal that affects their reputation.

Some of the members do various tasks for the government, such as conquering new islands for the World Government (described as "liberating") and protecting nations, but their ultimate purpose is to keep other pirates in check. Most importantly, the Shichibukai provide assistance to maintain the balance of The Three Great Powers.

Technically speaking, the Shichibukai are considered allies of the government, and in times of war are obligated to collaborate with the Marines. Still, many Marine officers despise them and consider the Shichibukai to still be pirates. Several high-ranking Marines expressed surprise that even two of the Shichibukai actually bothered to show up when summoned for the meeting about Crocodile's removal, and they were very shocked when a third arrived.