Royal

A Royal is a member of a kingdom's ruling family. The title is generally hereditary or acquired by marriage; however, rulers and ruling families can change via hostile takeovers or through the demise of all possible heirs. The royal family, or "monarchy", can consist of a King patriarch, a Queen matriarch, and Prince and/or Princess heir(s).

Royalty in One Piece
Most kingdoms in the world of "One Piece" are ruled by a royal family of some sort. Most of the rulers are monarchs, but they can go by a variety of names and titles: for example, "Empress" in Amazon Lily, "God" in Skypiea, "Duke" or "Master" i Zou, and "Daimyo" or “Shogun” in Wano Country. The patriarch of Kamabakka is referred to as "Queen" due to the culture of the kingdom.

The titles can be revoked with the replacement of the ruler by means of force, rebellion of the people, the monarchy abandoning their kingdoms, or by the the members of the royal family. Rulers who have abandoned their kingdoms and their descendants may claim the country's throne years after they left.

Every four years, the Reverie is held in Mary Geoise. It is a meeting of the current monarchs of World Government nations, during which the monarchs discuss various issues of world importance.

Some kingdoms have a group of people whose job it is to guard the royal family.

Royals
During the Void Century, twenty unnamed kings join forces to take down the Great Kingdom, and nineteen of them later left their countries to live in Mary Geoise, where they changed their titles to "Celestial Dragons".