Yonko

The Yonko are the four most notorious and most powerful pirate captains in the world (according to the World Government), but are neither allies nor enemies of one another, preferring to remain autonomous under most circumstances. These four individually reside within the second half of the Grand Line (known as the "New World"), exerting impressive influence and control over dozens of other pirate crews and self-governing islands.

Power
As an entity, the Yonko are one of the Three Great Powers of the world, the others being the Shichibukai and Marines. Their strength and influence are such that they live quite literally as though they are emperors, doing mostly as they please. Individually, they are considered to be the most powerful pirates in the world, as well as having a powerful crew. Because of their power, they can declare chosen locations as their own; the mere declaration is often enough for others, both Marines and pirates alike, to acknowledge their authority.

The Yonko themselves are not allied together as a coalition compared to the Shichibukai and follow their own individual paths. The possibility of two or more of the Yonko forming an alliance is enough of a threat to cause the World Government to panic, as seen when the World Government attempted unsuccessfully to prevent a meeting between Shanks and Whitebeard at one point in the series. The World Government feared that the two Yonko may unite together and create a stronger threat. A war between two of the Yonko is considered a terrifying crisis, as Shanks' interception of Kaido's advancement on Whitebeard had taken the Marines by shock and awe, putting the World Government in a "cold sweat" as stated by one of the Marines.

Because of their immense power, it is necessary for factions aiming to oppose them to first strike an alliance with other factions in order to bring the Yonko down. However, even with multiple alliances amongst powerful pirates, the chances of defeating a Yonko are still extremely low.