Sif is a non-playable character in Marvel: Ultimate Alliance.
Sif is a beautiful warrior-goddess of the Vanir. Heimdall, her brother, was the sentinel of Asgard's Rainbow Bridge, which connected Asgard and Midgard, the mortal realm. When Sif was a child, she possessed long golden hair. She often played with young Thor, Odin's son, and Loki, Thor's adopted brother. Sif and Thor fell in love while they were quite young. When she was kidnapped by Storm Giants and made a prisoner of Hela, Thor offered his own life to ensure her release. Hela, impressed by his nobility, released both of them. Later, the jealous Loki cut off all of Sif's golden hair while she was asleep.
Guessing that Loki had something to do with Sif's loss of hair, Thor angrily demanded that Loki restore her golden locks. The scared Loki contacted two dwarfish craftsmen, Brokk and Eitri (who would later create Thor's hammer Mjolnir), and asked them to create new hair for Sif. Loki was either unable or unwilling, to pay the dwarfs enough gold to create golden hair. The dwarfs decided that "for the price of naught" they would make "strands of naught." The dwarfs created magical strands of hair from the blackness of the night. The strands of darkness took root the moment they were placed on Sif's head and then began to grow out. Thor found that Sif's new black tresses made her even more attractive, disrupting Loki's plan to ruin their relationship.
For a time, Thor and Sif had a falling out. Many years later, Thor began his career as a superhero on Earth and had a long-time love affair with the mortal Jane Foster. After this, however, Odin arranged for Thor to meet the now-adult warrior-goddess, Sif. They became close companions and fell in love again. They promised to someday marry each other. Unfortunately, Thor's feelings of responsibility to the Earth and its people often came between the two lovers. In the past, Sif tried to adjust to living on the mundane Earth. Unfortunately, Sif preferred the majesty of Asgard.