Sif was a beautiful warrior-goddess of the Asgardian pantheon. Heimdall, her brother, was the sentinel of Asgard's Rainbow Bridge, which connected Asgard and Midgard (Earth).
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, 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 super hero 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.
At one point, Thor's former mortal lover, Jane Foster, was dying, and Sif merged her life force with Foster's, reviving the dying woman. Sif did this partially so that she could try to understand Thor's attraction to mortals, especially Jane Foster. Soon afterward, Foster was separated from Sif, when Foster was exiled to a limbo-realm called the Runestaff of the Possessor. Sif and Thor eventually rescued Jane Foster and reunited her with her mortal love, Keith Kincaid.
Thor and Sif later came to the aid of the alien champion, Beta Ray Bill, who was the guardian of a large space fleet that carried the surviving members of his alien race in suspended animation. Sif, Beta Ray Bill, and Thor drove off a horde of Surtur's demons that were attacking Bill's fleet. Beta Ray Bill and Sif found themselves attracted to each other while the war with Surtur raged on. During this time, Thor had fallen under an enchantment that caused him to fall in love with the Asgardian Lorelei. Thor was so blinded by the spell that he even struck Sif in anger.
In a final climactic battle with Surtur, Thor, Loki, and Odin defended Asgard. Meanwhile, the Ancient Casket of Winters had been opened on Earth. The warriors of Asgard, led by Sif and Bill, protected Earth from Surtur's demons and the renegade Dark Elves working with Surtur. Surtur and his forces were defeated on Asgard and Earth
Before returning to Asgard, Bill and Sif spent some time together on Earth. Once back in Asgard, Sif and Bill defeated the Titanium Man, the Russian villain, who was leading a group of disgruntled American war veterans wearing the Credit Card Soldier armored battle-suits.
Sif and Bill eventually returned to Earth. Sif finally came to forgive Thor, after realizing that Lorelei was to blame for Thor's brutality. Sif was also impressed by Thor's heroism when he entered Hela's realm to rescue some mortals' souls stolen by the death goddess. This also caused Sif to accept Thor's guardianship of Earth and Asgard.
When the Egyptian death god, Seth, attacked Asgard, Sif fought along with the other brave Asgardian warriors. During the battle, Leir, the Celtic god of Lightning, saw Sif and developed an attraction for her.
Later, when the Ice Giant Ymir attacked Asgard, Leir arrived in Asgard planning to ask Sif to marry him. A worried Sif agreed, once two conditions were fulfilled. The first condition was that Leir help her find Thor and bring him back to Asgard. Leir agreed and they traveled to Earth in search of Thor. The trail led from Earth to the Black Galaxy, where they finally found Thor. Leir got them back to Asgard in time to witness the final battle between Ymir and Surtur, which heralded Ragnarok.
Thor and Odin were able to permanently postpone Ragnarok, saving the universe. Leir then had to face Sif's second condition: to defeat Sif's chosen champion in battle. Thinking that Sif would ask Thor to be her champion, Leir was stunned when Sif acted as her own champion and easily bested the arrogant Leir.
Since then, Thor and Sif continued their close relationship.
When Thor was revived and began awakening the Asgardians, Sif's absence was notable. Hoping to find her, Donald Blake, briefly separated from Thor, sought out Jane Foster in New York City to see if she was again housing Sif's spirit. Finding Jane to be lacking Sif's spirit, Donald left the hospital where she worked, little realizing that one of the patients there was actually Sif, being held hostage in the body of an old woman named Mrs. Chambers who was suffering from what was thought to be terminal cancer in that hospital. Loki was reborn in the female body that was originally meant to be for Sif and used his sorcery to mask her from Thor's abilities, giving her a mirror that showed Sif her true form, but she was unable to let anyone know who she was.
Sif remained in the hospital inside her host, fighting to stay alive. When he could not find Sif, Thor began to wonder what would happen to the spirit of an Asgardian if their host died. Loki later revealed to Thor that Sif's spirit was too weak to answer Thor's original summons to the Asgardians and when Loki was in the process of transferring his soul from her current female form to his true male form, the process would kill Sif's host, and he should say goodbye to her.
Upon learning the truth from Mrs. Chambers, Jane Foster called Donald Blake and informed him that she had found Sif, with Thor successfully being able to restore Sif mere moments before Mrs. Chamber dies. With Sif resurrected, Mrs. Chambers passed away.
Sif later joined Thor in exile in Oklahoma and checked into the same hotel as Donald Blake under the name Sylvan. She was seen fighting alongside the Asgardians during the Siege.
When Odin sensed that the Serpent had returned, he hastily ordered all Asgardians to return to Asgard (The 'true' one rather than the ruins after Seige). When Thor fought back against the All-Father, she could only watch in shock as the Thunderer was defeated and taken to the realm in chains.
When Loki freed Thor (believing Odin is going too far to destroy the Serpent), Sif along with the Warriors Three, Balder and Hermod offered to take him back to Midgard to aid the Avengers, but not before Odin noticed what was going on. However, Thor managed to convince him to save Earth, and the All-Father promptly sent him there. But Odin gave Thor a warning; his time limit was up to the point where Dark Asgard would rise. With that, Odin promptly sent him to Midgard, even tossing him Mjolnir (which the All-Father had previously confiscated).
When Thor died, she attended his funeral.