Sif is the Norse Goddess of the grain, who is a prophetess, and the beautiful golden-haired wife of Thor. She is a swan-maiden, like the Valkyries, and can take that form.
By her first marriage to the Giant Orvandil, Sif had a son named Ullr ("the Magnificent"), who is a god of winter and skiing. By Her second husband Thor, She had a daughter, Thrudr ("Might"), a Goddess of storm and clouds and one of the Valkyries, and two sons, Magni ("Might") and Modi ("Anger" or "The Brave"), who are destined to survive Ragnarok and inherit Mjollnir from Thor (though some say the Giantess Jarnsaxa "Iron Sword" is their mother).
Sif is famous for her very long, very golden hair. One night, Loki, who just couldn't resist a little chaos and mischief, snuck into her chamber and chopped it all off. A sobbing and horrified Sif went straight to her husband, who in His rage started breaking Loki's bones, one by one, until finally He swore to make the situation right. So he went to the dwarves and persuaded them to make a new head of magic hair for Sif from pure gold. The new head of golden hair was given to Sif, where it magically grew from Her head just as if it were natural. Her golden hair is said to represent the wheat of summer that is shorn at harvest-time.