Hel is the queen of the underworld. She is the only daughter of Loki and Angrboda that we know of. When the gods learn of her existence, it is Odin who recognizes that she has tremendous power. Just like her siblings, her looks can only be described as “interesting”.
One half of her body looked like a regular girl, but the other half was a decomposing corpse. The gods gave her a world of her own to govern, the realm of the dead (Helheim). They thought this was the best way to rid themselves of the powerful young woman.
Hel makes an appearance in several stories of Norse mythology. She is the one that refuses to release Baldr to the realm of the living when it turns out that not all living beings would weep for him. Even the combined power of all Aesir (including Odin, Thor, and Frigg) is not enough to overrule her judgment in that instance.
The destiny of Hel is unknown, as it seems as if she does not take in the battle of Ragnarok herself. The Edda only mentions that all of her people (we are going to assume these are the souls that died a dishonorable death) fight alongside the giants. Then again, we also know that Baldr returns to the realm of the living after this battle has been completed. Could this be a sign that Hel herself has perished during Ragnarok as well?