Nephthys was the youngest of the children of Geb and Nut, born on the fifth of the five extra days. She was the wife of Set, but in her sympathies she was always with her sister Isis. The Egyptian myth held that Nephthys, longing for a child whom Set could not give her (for he was barren in some traditions, or refused her, or could only get the desert-creatures with her), disguised herself as Isis and lay with Osiris in his living kingship; from this union was born Anubis. When Osiris was killed, Nephthys joined her sister in the long search; the two of them together were the principal mourners of the slain god, and the Lamentations of Isis and Nephthys, sung antiphonally by two priestesses at every Osirian funeral, were among the most affecting liturgical texts of the Egyptian church.

Nephthys's name in Egyptian was Nebet-Hut, "Lady of the House," the house in question being either the temple itself (in which case she was the protector of sacred precincts) or, by extension, the funerary temple — for she was the mistress of mourning. In iconography she stood at the head of the bier, while Isis stood at the foot; in the canopic jar arrangement, she protected the jar that held the lungs of the dead. She was the gentler of the two sister-goddesses, the one whose role was less to remedy than to weep — and the Egyptian theological imagination accorded weeping a high dignity, for the tears of the goddesses were held to be themselves a kind of efficacious mourning that aided the dead's transition.