By re-reading Skypiea I noticed this panel: the four doors of the ordeals are not centered on the face of the statue and we can clearly see that there was a 5th door on the right, which seems to have been lowered. So it could have been a 5th ordeal and therefore a 5th priest.
Here is a better view:
Did Enel had a 5th priest? Another reason to believe it is the parallel between Enel and Im: like Im, Enel considers himself a God, controls the Holy Land, can annihilate an entire island in a single attack… The Priests of Enel would then be the equivalent of the Gorosei, but the members of the Gorosei are 5 and the priests of Enel are 4.
If Enel had a 5th Priest who could it be?
The most obvious answer is Urouge. Urouge is obviously from the celestial islands and he has the same wings as Enel’s priests, who come from Bilka. He’s also surrounded by religious symbols and called “high priest” by his followers. Urouge would have left Enel before the events of Skypiea to explore the blue sea.
The character could allow us to learn more about Enel’s past and about the celestial island of Birka, apparently destroyed by Enel 8 years ago.
*Theory by Albrede
