Her name, Nebet-hut, means "lady of the enclosure" or "mistress of the house" — the "house" being a temple precinct rather than a domestic residence — and she wears it on her head, the basket sign for neb sitting on the house sign, so that the goddess is literally crowned with her own name written out. Egyptian iconography does this often, and it means that where the figure is otherwise identical to Isis, the headgear is the only way to tell them apart.
That near-identity is the point of her. Nephthys exists as one half of a pair, and she is almost never depicted, invoked or narrated alone.
The two kites
Egyptian funerary practice placed two mourners at the body: one at the head and one at the feet. On coffins, on canopic chests, on the walls of burial chambers and on the corners of sarcophagi, the two goddesses kneel with arms spread, and they are shown as kites — the black kite, a bird of prey whose call is a long thin descending whistle that Egyptians evidently heard as a woman keening. The pair fly over the body of Osiris, search for the scattered pieces, fan the dead with their wings and lament.
The lamentation was performed. Papyrus Bremner-Rhind preserves the Songs of Isis and Nephthys, with a rubric specifying that two women with beautiful bodies be brought in, their names written on their shoulders as Isis and Nephthys, and that they recite the text before the god. The songs are direct address to the dead — come to your house, we do not see you, come back to us — and they are among the very few Egyptian religious texts where grief is expressed as grief rather than as ritual formula. Whatever theology surrounded it, an Egyptian funeral employed two women to stand at either end of a body and cry out for the man who was gone.
Her position in the family
She is the fourth child of Geb and Nut, sister to Isis, Osiris and Set, and married to Set. The marriage produces nothing: it is a deliberate barren counterpart to the fertile union of Osiris and Isis, and the family tree draws the symmetry, which is one of the clearest signs that the Heliopolitan Ennead was designed rather than accumulated.
When Set murders Osiris she sides against her husband, joins her sister, and helps recover and mourn the body. Later tradition — reported by Plutarch, a Greek writing around 100 CE, and consistent with some Egyptian material — gives her a child, Anubis, conceived with Osiris after she disguised herself as Isis, and then abandoned and raised by Isis. In Plutarch's telling Set discovers the affair from a garland of clover left behind, and it is his motive for the murder. That is a Greek storyteller's structure, and Egyptian sources give Set no stated motive at all; the older material simply has the killing as a fact.
Not a goddess of death
English summaries routinely call her a goddess of death, night and darkness, sometimes constructing a light-and-shadow opposition with Isis. The sources do not support the schema. She is associated with the night in some texts and with the west, but her recurring functions are protective: she guards the mummy, she shields the canopic jar of Hapy — the baboon-headed son of Horus containing the lungs, not to be confused with Hapi the inundation god — she stands at the corner of the sarcophagus, and she accompanies the sun's night barque. In the Pyramid Texts she assists the dead king's ascent.
She also appears with Isis flanking the sun disc, or flanking the djed pillar, or supporting the king. She is a supporter, in the structural sense: an Egyptian composition frequently needs a matched pair at either side, and she is the one who makes the pair.
Cult, or the absence of one
She has no great temple. Nothing answers to Dendera or Philae in her case: no destination for pilgrims, no annual round of feasts held in her name. She is attested in shrines within other gods' precincts, notably in the Osirian complexes and at Komir in Upper Egypt where a local goddess was assimilated to her, and she is present in every funeral in the country. This is a deity whose worship was ubiquitous and institutionally invisible, which is a different thing from being minor.
Her Greek reception folded her into the Isis mysteries as a subordinate figure, and Plutarch attempts to systematise her as the barren land at the edge of the flood against Isis as the fertile ground — an allegorical reading that tells you a great deal about Plutarch's Platonism and very little about Egypt.
Modern reception has been thin and, where it exists, has generally inverted her: contemporary fiction and popular occultism tend to make her a dark or death-aligned counterpart to a light-aligned Isis, which is the same schema the sources do not support, arriving by a different route.
Questions about the second sister
Why would a goddess side against her own husband?
The question assumes the marriage is the primary bond, and in this material it is not. The generational sibling group of Geb and Nut's four children is the structural unit, and Nephthys is inside it on the side of her brother and sister. The Egyptian arrangement of the Ennead marries siblings to siblings precisely so that no alliance can be explained by marriage alone. Her loyalty runs to the murdered brother rather than to the murderer, and no text treats this as requiring explanation.
Is the "disguised as Isis" story Egyptian or Greek?
Both, partially, and this is a good test case for reading the sources honestly. Anubis as a son of Nephthys is attested in Egyptian material, and Egyptian texts also know his older parentage as a son of Ra. The specific narrative of the disguise, the seduction and the abandoned infant comes to us through Plutarch, roughly two thousand years after the earliest strata of the myth and from someone who never lived in Egypt. The relationship is Egyptian; the story explaining it may not be.
What does the kite have to do with mourning?
Black kites are common in Egypt, they are scavengers that gather where a body lies, and their vocalisation is a thin, drawn-out, repeated cry. Egyptian ritual took the bird that arrives at a corpse and made it the form of the goddesses who arrive to grieve — the same conversion of an unpleasant natural fact into a protective figure that produced the jackal god of the cemeteries. The Egyptian tradition hub has more on that habit of thought, which is one of the most distinctive things about the religion.