In Disgaea, there have been several characters with the title of Overlord, especially in Disgaea 5.
Overlord is a demon that rules the Netherworld. So that would mean there is only 1 overlord per Netherworld. Usually Overlords are very powerful but it seems like that is not always the case, for example Usalia and Seraphina’s father are no where near the level of Zetta or Zenon.
This also means that powerful demons are not automatcially given the title of an overlord. In Disgaea 2, Etna was in Veldime so she too can have the Overlord title. She was strong enough to be an overlord, but Laharl was the Overlord where she was at.
Rarely, there are demons who has title exceeding overlords. Their title is usually Tyrant Overlords or Super Overlords. Valvatorez and Killia has had the title of “Tyrant” but when people hear that name, they are usually thinking of only one demon, Tyrant Overlord Baal.
In games like Disgaea 3 and 4, they mentioned that the Dean of Evil Academy and the President of Corrupterment has same status as overlords. So it seems like overlord is more of a status than anything.
I hope you enjoyed this!