Thor married Sif whom he loved both tenderly and dearly. A goddess of regal beauty to rival Freya, Sif took great care of her gorgeous golden hair which she treasured. Loki found her asleep and decided to take advantage of it. He cast a spell on Sif to keep her from waking up and in the meantime, he cruelly sheared off every strand of Sif's hair directly at the roots so it couldn't grow back, until she was bald. Sif was horrified upon waking up and when Thor found her in such distress he was outraged. He seized Loki and threatened to destroy him when the trickster god offered to make his wife a new head of hair, far more beautiful than her original hair. Thor allowed this, and off Loki went to Svartalfheim, the world of the dwarves. The master craftsman forged the perfect wig of flowing, floor-length, lustrous hair made of real gold. When it was placed on Sif's head, it became one with her flesh and the hair grew just like real hair.
Sif is mentioned twice in The Sword of Summer. First, her name is seen tattooed on her husband Thor's arm. Thor later references the time Loki stole her golden hair. Magnus mentions that she looks like a dryad spirit that his cousin Annabeth told him about.
Sif appears to Magnus and his friends when they are being chased by the mountain giants. She saves them from being killed and brings them to Bilskinir. She gifts Hearthstone with a leather pouch of runes made from a rowan tree, but warns him that one rune is missing. She then sends them down to her husband, Thor. Then she gives then guest rooms in the upper levels of Bright Crack.
The next morning, she appears and gives Alex Fierro, Samirah al-Abbas, andMagnus Chase clothes for the wedding. She then sends them through a rowan tree portal to the Taco Bell. Alex Fierro gains a liking for Sif and makes a trophy for her.
Sif once had beautiful long golden colored hair, before Loki cut it off and was replaced with real gold hair. When she appears to Magnus Chase and his friends, she is wearing an orange-red dress clasped at her shoulder with a silver and green brooch. She has fine elfish features and had long hair of red gold which is described as rich, warm, and lustrous.
She presumably possesses the standard abilities of a goddess
- Geokinesis: As the Goddess of the Earth, Sif has divine authority and absolute control of the earth.
- Chlorokinesis: As the Goddess of Grain and Harvest, Sif could grant fertility to the earth, allowing plants to grow where they could not grow before, and even aided in the growth of plants simply by being near them.
- Her magical hair is attached to her head just as though it was her original one. She can weave her gold hair into any material object. The hair she weaves magically grows back. She often uses this to pay off expenses.
- Sif was well known for her golden hair.
- Sif is portrayed as a vain goddess who cares mostly about her appearance, but there's a sweet side to her as well as shown when she saved Magnus's group, gives Hearthstone a bag of runes, and prepares Magnus, Alex, and Sam for the wedding in The Hammer of Thor and when she embraces Alex following the victory against Loki in The Ship of The Dead. She even seems to have developed a degree of affection for Alex as well.
- Her Greco-Roman equivalents would be Demeter/Ceres and Gaea/Terra.
- Sif Mons, a volcano on the planet Venus, is named after her.
- According to Sam the fairy tale Rapunzel originated from the golden hair of Sif.