History
The being that would come to be known as Sif began its life on a distant world manipulated by the Celestials. As part of their method of reproduction, the Celestials imbued Sif's world with an embryo and manipulated the genetics of its inhabitants to grant them the power to defend their world, and thus the embryo within it. Sif and her people eventually reached a point in their mutation where they lost all shape and definition, instead becoming defined by the perceptions of others.
Escaping their world at some point, they made their way to Earth, where they encountered the ancient Norse. The Norse, and in particular one among their number, a storyteller known as Donnerson, shaped Sif and the other Asgardians by believing them to be the gods of their myths come to Earth. Donnerson would merge with two of the Asgardians and assume the role of Odin.
It was then that Sif assumed her current form, as a warrior woman and beloved of Thor.[1] [2]
Much of Sif's subsequent history presumably mirrors that of her Earth-616 counterpart.
Following the battle against the final host of Celestials, Loki and Thor learned the truth of their origins and conspired to bring down Odin and free their people from his story. When Odin's lies were revealed, Sif would briefly return to her previously undefined form with the rest of those of Asgard, but she and the rest of her people ultimately chose to return to Odin and continue acting out his fantasies.[3][2]