The Demon King is a hereditary title given to the Supreme Ruler of the Demon Realm. To distinguish them, each Demon King receives another epithet; for example, the Demon King seen in the story is known as "Ruby Eyes."
As the Demon Realm is largely a kratocracy—a government where the strongest seize power—most past Demon Kings have been extremely powerful, and the current Demon King is an anomaly in her lack of combat prowess.
Each new possible Demon King successor acquires what is known as the "seal of kings," placed somewhere on their bodies; the current Demon King, for example, has it between her cleavage, while the spirits of past Demon Kings were shown to have it located on their foreheads or their chests. This seal is placed on anyone who decides to contest for the position of Demon King, though it is never mentioned what happens to those who fail to win the position. (In all likelihood, the successor is the last one standing and any other competitor is dead.)
The victor gains all the memories, wisdom, and power of past Demon Kings, but at the price of their sanity and reason. Even otherwise-peaceful people will be reduced to power-hungry, battle-lusting individuals. It is also later revealed that the spirits of past Demon Kings can choose a new host if they find that the current Demon King is unsuitable.
- When asked by Hero why the "Demon King" is a woman, she answered that as hereditary title, they use "King" regardless of the title-bearer's gender. (In the Wowmax Media! translation of the Akira Ishida manga adaption, they translated her name as "Demon Queen" regardless.)