The Four Emperors are four pirates considered to be the most notorious and powerful captains in the world. Currently, these four are recognized as Shanks, Marshall D. Teach, Monkey D. Luffy, and Buggy, with known predecessors being Edward Newgate, Charlotte Linlin, and Kaidou.
Historically, the Emperors have dominated the second half of the Grand Line (the “New World”), captaining powerful subordinates and massive fleets while exerting influence and control over dozens of separate pirate crews and even full-fledged nations. As such, they are viewed by the World Government as one of the Three Great Powers integral to world stability. However, no formal ties exist between them; each Emperor works as an independent force, and may oppose or align with whomever they see fit.
