Ren

A being's secret name, or ren, is a name that belies the nature of the person's soul. A secret name is typically only shared in times of great need or as a gesture of deep trust (by revealing one's secret name to another, they gain power over the owner). The owner would then be forced to do as the holder demanded. The name cannot be revealed by anyone, except by its owner or by the one closest to his or her heart. Also in the Throne of Fire, Set told Sadie (who figured out his name in The Red Pyramid) that she could simply give up that knowledge. Jaz told Sadie in the same book that the ren is 1 of 5 aspects of the soul.

Set
Set's secret name is Evil Day. Sadie originally found out his name, which she used to banish him to the Duat in The Red Pyramid, but gave it up in The Throne of Fire in Russia.

Carter
Sadie finds out Carter's has been playing with her all this time. Only Sadie knows it and I wish it told us in The Throne of Fire. His secret name is dick head.

Jaz
When Sadie's ba drifted when she was asleep, she learns from Jaz that a secret name is also known as ren. Sadie first thought that Jaz appeared to her as a ba but Jaz told that she is appearing to her as a ''ren. ''Later she comes back from her coma by Ra's sun.

Bes
Carter, Sadie, and Bes play a game with Khonsu in the Throne of Fire to win moonlight to create 3 extra hours, but if Khonsu won he would eat their souls. Later Khonsu won 1 stage and Carter was going to sacrifice himself but Bes did and Khonsu devoured Bes's ren. Now Bes is in a retirement home in the Duat called Sunny Acres/The Fourth House. Bes is also the car driver going to the hotel.