Set, also spelled Seth or Sūtaḫ, is a deity from Egyptian mythology. Set is the deity of deserts, storms and chaos and served as one of the greatest deities in Egyptian myth. Set was married to the goddess Nephthys, who was also his sister. He killed and dismembered his brother Osiris out of jealousy. Isis, Osiris' wife and sister, reassembled him and Osiris became the ruler of the dead. Isis gave birth to Osiris's child, Horus, who became an enemy of Set. In earlier myths, Set fought Apep each night as Ra journeyed through the underworld. Set had been demonized to some extent, and the Greeks interpreted him to be a force of evil, which may be one of the reasons he is viewed as an antagonistic force in the original novels.
Seth was one of the earliest major Egyptian deities to be demonized. Already associated with the uncertainties and hardships of the open desert, he had a bad, but not ignoble reputation. Due to Set's ties with his mortal enemy Apep, the two became ironically intertwined. Because of this, especially in the western world, conceptions of Set began to take on the serpentine features of Apep. This is further confounded by the inability of archeologists to identify a specific animal-form to Set, so instead the label "Typhonic" was adopted, like-Typhon of Greek mythology, which is to say of many forms, primeval, and generically serpentine. It is not impossible to see how it was Set became likened to serpents, even while no such relationship exists. It is even possible because Set's animal-head often adorns staves, the long slender snake-like shape of a staff was attributed to Set, such as when Moses transmutes a staff into a snake in the Old Testament. And even in modern times images of Set have been probably more influenced by the fictional demi-god 'Set' of Conan fame than any actual representation of Set. All of this comes to a head in the second Digital Devil Story novel which features Set, Typhon, and Apeps (plural), and in league with one another.
Set only appears briefly at the end of Digital Devil Story: Megami Tensei. Ohara accidentally summons him while attempting to contact Loki. He is insulted that he was summoned yet Ohara was calling for Loki instead. Set's appearance is mostly foreshadowing of his role in the second book.
Seth acts as the final boss of the main episode's storyline. The first encounter is designed to fail with either the player running out of time or dying. Seth was originally Setsu, who transforms into him during Act 19. They are then given a ring to boost damage against them by the leader of their respective alignment. The final fight against him within the main story is a timed encounter. After completing side a of the extra chapter, players are able to follow the side b quest where they are brought back to the place of their first encounter with Seth to fight an even stronger form. This battle against Seth takes place in a void above the earth, surrounded by stars and meteorites and is comprised of three different forms, ending with a timed fight against its final form. By defeating Seth, players are able to obtain items to acquire his plug-in. He is a triple fusion of the three alignment act bosses leading up to his fight-- Chimera, Isis, and Gabriel.
Seth appears in the Challenge Quest, Isis' Search for the Coffin. He steals Osiris coffin and tosses it into the waters, which brings it to Naraku. He attempts to stop Flynn from delivering the Sarcophagus to Isis and respawns even if he is defeated. He is able to stop Isis from resurrecting Osiris before the Challenge Quest Osiris Resurrection by stealing Osiris body and tearing it apart and scattering it across the desert areas of Tokyo. He attempts one more futile battle against Flynn after he retrieves Osiris parts to prevent Osiris Resurrection, but is defeated.
Seth returns as a Persona of the Moon Arcana in Persona 3 FES, retaining his attributes. In the playable epilogue of FES, titled The Answer, Aigis can also summon Seth by fusing the appropriate Personas in the Velvet Room.