A ren, also known as a secret name, is the true name that states the nature of the entity'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, the holder gains 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. 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. The ren is one of five aspects of the soul alongside the ba, ib, sheut, and ka.
Set's secret name, as told by Zia, is Evil Day. Sadie used his secret name in The Red Pyramid to almost banish him to the Duat. In The Throne of Fire, Sadie gave Set back his secret name in exchange for information.
Sadie finds out Carter's secret name in The Throne of Fire to save him from the tjesu heru's poison. Later in the book, Carter says that he should change his secret name to "Embarrassed to Death by Sister", due to being shocked and embarrassed that Sadie knew him better than anybody else in the world. Sadie later displayed the ability to make him hit himself in the face as a joke and he indicates she may do this a lot.
When Sadie's ba drifts when she is sleeping, 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.
Carter, Sadie, and Bes play the game senet with Khonsu in the The Throne of Fire to win enough moonlight to create three extra hours, but they would have to sacrifice one soul for every stage Khonsu won. When Khonsu won a stage, Carter was going to sacrifice himself, but Bes offered his own ren for Khonsu to devour. He is not seen again until near the end of the book, where he is in a retirement home known as Sunny Acres/The Fourth House in the Duat. In The Serpent's Shadow, Sadie comes up with a plan to retrieve Bes' sheut and send it back to him, believing that this would restore Bes the same as destroying Apophis' sheut would destroy him. Upon finding and retrieving the sheut Walt Stone binds it into a shabti and then has Sadie recite the reverse of an execration spell on the sheut. This successfully restores Bes to normal.
After Isis poisoned Ra, she tricked him into telling her his secret name to relieve him the pain. Isis did so, but Ra was forced to leave to the heavens, partly out of humiliation as well. Later, Isis masters his true name and makes Osiris the king of the gods, forcing Set to plot against him.
Sadie gives Annabeth Chase her secret name to allow her to practice Egyptian magic as part of their plan to defeat Setne. After the battle, Annabeth tells the others that she is forgetting all the magic she learned, but its unclear if she also forgot Sadie's secret name.
- The reference of a secret or true name is used in several different series, such as the Inheritance Cycle.
- Sadie tells Annabeth Chase her secret name to help defeat Setne.
- Set and Shezmu are the only ones whose secret names are actually revealed.