Demon King

The Demon King is the current ruler of the Devildom, but resides at the bottom of the Devildom as he has no interest in the affairs of the three worlds. His son, Diavolo, rules the Devildom in his place. However, Diavolo has yet to be crowned as the next king.

Story
Note: this section may include spoilers

Main Story
The Demon King was responsible for the brothers' ranking and "age" when they fell from the Celestial Realm, with Lucifer being the "oldest" and most powerful and Belphegor being the "youngest" and the weakest. Despite being the actual youngest in age, Satan was deemed the fourth oldest due to his power.

According to Luke, he has not seen the Demon King since a ceremony 1,000 years ago. This could have been his last public appearance.

The Demon King did have a wife, who died while giving birth to Diavolo.