According to the thirteenth-century Icelandic scholar Snorri Sturluson, she’s the daughter of Loki and the giant Angrboða (“Anguish-boding”), and therefore the sister of the wolf Fenrir and the world serpent, Jormungand. But Snorri can’t be taken at face value, and this family tree is likely something he himself invented in the interest of making Norse mythology seem more neat and tidy than it actually was.
Hel is generally presented as being rather greedy and indifferent to the concerns of both the living and the dead, but her personality is little-developed in what survives of Old Norse literature. She’s mostly mentioned only in passing.
Due to how ill-defined her character is, many scholars view Hel as a late feature of Norse heathendom, and likely an invention of the poets. Either way, it’s interesting to note that the Old Norse word Hel, which is the simpler and probably much older version of the place-name Helheim, is grammatically feminine. The writers cited above use this as a piece of evidence for the argument that the goddess Hel is a literary “personification” of the underworld. However, in the animistic and pantheistic worldview of the ancient Norse and other Germanic peoples, where nothing is inert or impersonal and everything is in some way divine, the view that the underworld comprises the tangible manifestation of a goddess – or, to put it another way, the view that a goddess is the force that animates the world beneath the ground – would be considered perfectly normal. The entire concept of “personification” would have been out of place.
Even if Hel is a literary invention – which must remain an open question – she’s a literary invention that’s very much in keeping with the spirit of the pre-Christian Germanic outlook on life.
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 Orel, Vladimir. 2003. A Handbook of Germanic Etymology. p. 156, 168.
 See, for example:
Ellis, Hilda Roderick. 1968. The Road to Hel: A Study of the Conception of the Dead in Old Norse Literature. p. 84.
Simek, Rudolf. 1993. Dictionary of Northern Mythology. Translated by Angela Hall. p. 138.