Usually just a back up player in Thor's adventures in the comics, in Norse Mythology, Sif was a goddess, Thor's wife and the mother of his children.
Well at least she did manage to get into both Thor films and show up on Marvel's Agents of S.H.I.E.L.D. (played by Jaimie Alexander).
The hair-cutting thing was worked into Marvel's story, but in the comics, her hair is replaced by black enchanted hair made by dwarves. Yeah....I know.
Also in the comics, Sif is the sister of Heimdall (aka Heimdallr in the myths).
Sif has your standard Asgardian powers, super strength (she can lift around 30 tons), durability and she's basically immortal. She's six foot two and weighs 425 pounds. 425 pounds? Yes, like all Asgardians, Sif's body tissue is about three times as dense as a humans. Sif also has the power of teleportation through time and space
She also carries a sword that has been enchanted by Odin that allows her to cut open passageways between dimensions.
Of course Sif was portrayed on the big screen by Jaimie Alexander in both Thor and Thor: The Dark World. And she also showed up as Sif on an episode of Marvel's Agents of S.H.I.E.L.D.