She's the daughter of Loki and the giantess Angrboda, and sister of the sea serpent Jormungandr and the monstrous wolf Fenrir.
Notable Stories Involving Her:
She's oftentimes described as a fiercesome hag, indifferent to the pleas of both the living and the deceased. When she was younger, she and her brothers were taken from their parents by Odinn, who feared they would all grow to cause trouble for him and the other gods. While Fenrir was chained and Jormungandr was thrown into the sea, Hel was tossed into the underworld, also called Hel. There, she receives those who have died of sickness or old age, or are just plain evil, while Odinn and Freyja divide up the slain warriors of battle between them. After her father Loki had tricked the blind god Hodr into killing his brother Baldr, Odinn had sent his son Hermod to ask her to resurrect Baldr. Hel agreed, but only if every thing in the world, both living and deceased, wept for him, to prove he was missed. Unfortunately, Loki was the only one who didn't weep, and thus, Baldr remained in her realm.
SOURCES: Wikipedia, Encyclopedia Mythica, & The Ultimate Online Resource for Norse Mythology and Religion