Hel = daughter of Loki. As one story goes, when Hel was born Óðin threw her into Niflheim and gave her control of nine worlds. There she was told to welcome all men who die from disease or old age. It is said elsewhere that she resides in a place which also goes by the name of Hel.
Garm = Hel’s hound. In earlier myths, this may have been Fenrir under a different name. At Ragnarök, Garm breaks loose from the underworld and kills Týr, but doesn’t survive the fight.