Im is an extremely high-ranking individual in the World Government and the actual holder of the Empty Throne whom even the Five Elders bow down to.
The full scope of Im’s abilities are unknown, but the fact that they sit on the Empty Throne, a seat whose occupant would be associated with sole authority of the entire world, and the fact that the Five Elders, who are publicly considered to be the highest authorities of the World Government bow before them and address them with a great honorific, would indicate that Im has the greatest known authority within the World Government.
So everyone is calling them Im or Imu in translations. I’m about 95% sure that this is a reference to the Canaanite god of the sea Ym (Yam), who rules in the abyss of the oceans, and who fought Baal before the creation of the world. In Canaanite language, Ym literally even means “sea”, and I can’t think of a more appropriate name.
Ym fought Baal for supremacy before the creation of the world and wanted to rule. This would fit extremely well with a theory that Ym (or Ym’s ancestor) was the victor who sought to erase the content of the Void Century (they warred before creation, in the void) from existence, and created the world as it is today.
As such their name is probably also actually just Ym in English.
*Theory by galleom5