Lady Sif and her brother, Heimdall, were born of the second generation gods of Asgard, although their parentage has yet to be revealed. During their childhood, Sif was a constant companion of Thor, Balder, and Loki. Young Sif would often spend time with Thor and Balder, adventuring, training, fighting and laughing. One particular source of amusement for young Sif would be watching Thor fruitlessly struggling to lift the mighty enchanted uru hammer Mjolnir. She would still empathize with the son of Odin when he failed to lift it, pointing out that he may still wield the weapon in the future and that she laughs at his inability to give up. The three would discuss Mjolnir and Sif would point out how she didn't like how the inscription on Mjolnir specified the male gender when she might be just as worthy to lift it one day. Balder and Thor would agree with her idea, possibly not wishing to incur Sif's wrath.
During her early years Sif's great talents as a warrior were not appreciated by many because she was a woman. Thanks to her friend Thor, who believed in her capabilities with a sword. Sif became one of the most appreciated warriors of Asgard, along with the Warriors Three, Loki, and Thor himself. As a child, Sif was born with luscious golden hair; like most other Asgardians. Loki decided to play a trick on her in retaliation for Thor preference of Sif’s company over his. As she slept, Loki cut off all of her golden hair. Replacing it with an enchanted wig, 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.
Sif would never forgive Loki for doing such a thing like that to her, this has created a long resentment and animosity between the two, even to adulthood. Sometime in her adolescent years Loki had team-up with an enchantress; Amora not so fond of Lady Sif herself. Amora the Enchantress plan would be to form a union with Loki to play and generally wreck havoc for Sif, Enchantress upfront about her jealousy towards the raven haired warrior and Loki getting a chance to exercise his strong dislike.Thor would lean in and Sif would mirror his body movement as both leaned in to each other for a kiss, before they would be be suddenly interrupted by the Enchantress. Enchantress would insult Sif quite directly and rather cruelly prompting Sif to stand up and slap Amora swiftly.
Thor would attempt to intercede and get both girls to calm down and relax. Sif would find this wholly annoying behavior and leave angrily leaving Enchantress smirking and Thor confused about what had just transpired. To make things tougher for Sif, Enchantress would interrupt her traversing the training grounds, and plant the idea in Sif's mind that Thor was tired of her and preferred the company of other females, causing Sif to sulk away.
Sif reflects on recent events by a fountain, when she is approached by Loki. Loki and Sif never having a good relationship between each other would cause an already downtrodden Sif weary of the trickster. Loki would inquire about Sif's feelings towards Thor prompting a string reaction in Sif. Loki would try and convince Sif that he empathized with her knowing something of jealousy himself. He would sit beside her and sympathize over Enchantresses harsh words revealing he had overheard that conversation the two young woman had. Loki would continue sympathizing with Sif, acting unusually kind and amicable with Sif, but Sif would still remain cautious and with good reason and something she would tell Loki in an attempt to get him to just admit he hated her.
Sif was also romantically involved with a warrior named Haldor, who was later enthralled by the sorceress Lorelei. The event led to Haldir's death, creating a deep hatred between Sif and Lorelei. Who Sif wouldn't mind marking her off of her hit list.
Eventually The day of Thor’s coronation arrived and it was clear it was going to be a great celebration; though there were many that believe that Thor was not yet ready to become king. Sif was among those that didn’t believe he was ready but she was not about to speak against the All Father and only hope that her friend would not let the crow inflate his head to much; though she would be there to call him an idiot and deflate it for him if needed. The celebration however was cut short, and the crown of Asgard wasn’t passed down to Thor, when a group of Jotun broke into the vaults. Turning a day that was supposed to be full of celebration into one of fear and unanswered questions.
When the Frost Giants of Jotunheim interrupted Thor's coronation ceremony by attempting to steal the Casket of Ancient Winters, the coronation was cancelled, much to Thor's anger.
as expected, and no one could blame him, Thor was angered and his friend; the Warrior Three and Sif, sought Thor to comfort him. It was in a dining hall that they not only found the crown prince but the younger prince Loki; already setting to task comforting his brother; the room of course showed great signs of the tantrum that Thor had thrown. Their attempts to comfort, and maybe even to provide a distract in ways that only friends could, turned to the plan to journey to Jotunheim to demand answers. Sif and the warrior Three did not fully agree with this idea at first, Sif even taking the moment to remind them that this course of action was forbidden to them. Thor however convinced his friends that this would be but another grand adventure and the six of them rode off to the Bifrost were Heimdall stood guard.
After convincing Heimdall of their quest they arrived in the barren frozen land of Jotunheim and were greeted by Laufey; the King of the Jotuns. Answer were demanded but none were given, and the six Asgardians were about to leave in peace when Laufey mutter an insult to Thor. That was the beginning of a great battle between the six of them and the Jotuns. Though they were not supposed to be there, the fact that Sif was in battle did not mean she could not enjoy it, until a friend become hurt. Fandral become impaled and the realization of their situation dawned on Sif and the others; demanding Thor that they leave now and calling for the Biforst to be open, while being chased by a ice hound that Thor saved his friends from. Surrounded by Jotuns, the gate would open and Odin would arrive setting on his steed known as Sleipnir.
Odin retuned the group to Asgard, Sif and the warrior three were ordered to leave; taking Frandel to the healing wing. It would be after the friends were once again gathered in their meeting room that they learned of Thor’s banishment, all of them were shocked at the punishment but knew there had been no way to have stopped Thor. Together the friends talked about what happen and it was reviled that Loki had alerted the guard to where they had gone. Sif asked Loki to go to the All Father and beg for Thor’s return, Loki argued that bring Thor back would not be good and left. Sif made the comment to the other that Loki had always been envious of Thor and even though he had saved their lives she; as well as the others; couldn’t help but suspect there was something else under all this mess. After all Laufey had made mention of a traitor in the house of Odin and being gifted with magical talents Loki could certainly be considered a threat.
The friends were not happy setting by waiting for the outcome and decided to go to the All father and request that Thor be returned, however they were greeted with the realization that the Allfather had fallen into his Odinsleep and Loki was now setting on the throne of Asgard. Sif requested an audience with Frigga but her request was refused when she was told that the queen would not leave her husband’s side at this time. Reluctantly the four knelt before Loki, Sif clearly showing reluctance even more so as she was the last to kneel and though she placed her arms crossed her chest in request she did not look away from Loki; giving him clearly the look of distrust she saved only for him.
Keeping a civil tongue Sif requested that Loki bring an end to Thro’s banishment to which Loki denied; stating he would not undo the last order that Odin had given. Adding insult to injury stating that bring back Thor would only make matters worse; and if it had not been for the warrior Three Sif would have slapped the smirk off Loki’s face. With a last look Sif turned and left with her friends, returning to their common room where it was clear that Asgard was in danger as long as Loki sat on the throne and the only way to save it was to bring Thor back. It was not a simple task however, for to leave Asgard at this time would be an act of treason; but the four friends felt there was no other way.
In the middle of planning what they would do, Sif and the Warrior Three were summoned by Heimdall and Sif was certain her brother; who saw all; would be forced to imprison them for their plans of treason. Before they could even explain what they were going to do Heimdall left the Biforst with the gate open and the group travel to Midgard to find and bring home their friend and prince.
It was clear that Midgard had changed since the last time they had been there, a few hundred years or so. However the troop soon found their friend having breakfast with his new found friends on Midgard. Introductions where quickly made and Thor was informed that they had come to take him home; to which he replied he could not and that he believe Odin was in fact dead. Sif quickly explained that Odin was still alive and that Asgard was on the verge of war; not to mention that Loki was setting on the throne of Asgard.
Unknown to Sif and her friends, their exit from Asgard had not gone unnoticed by Loki and while they were in the middle of their reunion with Thor their attention was drawn to the cloud formation that indicated the Bifrost was once again open and someone; or something; had arrived. The Destroyer had been sent to keep Thor from returning to Asgard. Thor and his Midgardian friends worked together getting the people out of town quickly as possibly, while Sif and the Warrior Three attacked. With the Destroyer distracted Sif leapt form a building and drove her blade into his heart; moments later however she was blown off as the destroyer as it turned around and fired at her. Sif quickly got to her feet again, ready to sacrifice herself to ensure the safety of the others; she would be proud to die such a death.
Thor stopped her from making such a sacrifice; Sif, the Warriors Three and Thor's new friends retreated away from the destruction, but Jane insisted on waiting to see what Thor was going to do. But Sif already knew; Thor would sacrifice himself to the Destroyer.
Thor went before the Destroyer and offered his life to save the people of Midgard. The machine, at the hand of Loki, struck Thor and fatally wounded him.Unbeknownst to the Warriors, Thor's sacrifice was known to King Odin in Asgard. Thor had proven himself worthy; and Mjolnir shot free from its waiting place and returned to its owner; no longer mortal and with his weapon at his side, Thor had the power to get rid of the Destroyer.
With the present threat gone, the Asgardians hurried to return home to deal with Loki. Sif and the warrior Three were not present at the fight between Thor and Loki on the rainbow bridge but would hear the details of it later. All of Asgard felt the destruction of the Bifrost as Thor smashed the rainbow bridge that held it to Asgard; saving the realm of the Frost Giants from destruction.
It wasn't until father and son returned that Asgard learned of Loki's death, vanishing into the abyss of space. Even at the lost of Loki all of Asgard celebrates Thor’s return, Sif however could feel the distance in Thor, even though he did well to act as if nothing was wrong. She knew that he longed to return to the Midgardian woman he had gotten to know while he was banished; as well mourn for the lost of his brother. Though it hurt to see that after all this time he did not feel for her the way she did for him, Sif was deeply sadden to see the pain that Thor was in over what he had lost.
Things fell quiet across the Nine Realms for some time, peaceful. Until, that is, a dangerous piece of Asgardian technology turned up in Midgard. The Tesseract, which the agents of SHIELD believed to be a power source. More dangerous was the person who possessed the item, by the time the Tesseract was in Heimdall's sight; Loki.
With the news that his brother lived, Thor was to set off to Midgard and retrieve both him and the weapon. Asgard waited for the valiant return of Thor and the betrayer, Loki, he would be bringing back.
After a great battle and an attack on New York by the Chitauri, Thor returned with Loki and the Tesseract in his possession. Loki was sentenced to Eternal Imprisonment, spared death only by the will of Frigga, and many of Aesir to including Sif soon moved on with their lives leaving the memory of Loki in their past.
Soon the Bifrost was repaired, causing the five friends to once again travel around to the Nine Realms to bring them back to order again. Sif rode into many battles and returned home victorious, though she could not take all the credit as the Warrior Three were ever at her side; not to mention Thor stepping in to lend a hand as well.
While celebrating their victory in Vanaheimr, Sif noted that Thor was not enjoying the celebration as much as he once had, and questioned him in private. Truthfully, all of Asgard knew that Thor was so focused on the woman he loved so far away. Sif tried to remind him that he needed to be focused on more than just Jane, even asking him to have a drink with her; perhaps hoping to finally find some way to share her own feelings toward him. But, as it so often seemed, her words had little effect on the prince.
During one of his visits to Heimdall to check on Jane, Thor found that Heimdall could not see her and so traveled to Midgard to assure her safety. It was there that Thor witness that something was wrong with Jane and brought her back to Asgard in order to find out what was wrong. The force was discovered to be the Aether, a weapon of the Dark Elves long since thought to be destroyed.
A group of Warriors returned from battle with prisoners who were locked in the dungeons of Asgard. One of the prisoners, a Dark Elf in disguise, broke himself and dozens of other prisoners out. Dozens of warriors hurried to the dungeons to battle the escapees. On her way to return order to the dungeons and stop prisoners Sif was shocked and displeased to see that Jane had come to Asgard.
Escaping prisoners however would become the less of their worries, as soon the Dark Elves attacked Asgard. Though the kingdom fought well in the end hundreds of lives were lost to including that of Queen Frigga. In order to protect themselves Odin ordered the Bifrost closed and that no one was to enter or leave Asgard. Thor knew that Jane, and Asgard, best hope was to get the Aether off Asgard and therefor with the aid of his friends came up with a plan to escape with Jane. Sif knew well enough that the plan was treason against the King; but Thor was adding a risk by including Loki in his plot.
Sif and the warriors all agreed on the plan, and it was quickly set into motion. Sif went to the room where Jane was being held and, after knocking out the guards watching her, brought her to where Thor was waiting. When the guards came for Thor, Sif told Thor to leave that she would give him time to escape; after threaten Loki that she would kill him if he hurt betray Thor.
Thor, Loki and Jane escaped Asgard and entered into Svartalheim. From that point on, Sif and the others had to trust that Thor's plan would work. Back on Asgard Sif and the others would learn of Loki’s death; as apparently risked his life to protect Thor; as well as what happen on Earth after Malekith had gained control of the Aether. In the end Malekith was destroyed and the Aether was captured; to which was later entrusted to Sif and Volstagg to deliver to a man known as the Collector.
On Thor's return, he told Odin that he wished not to take the throne. He would protect the realms, but he wished to stay on Earth with Jane. When his friends learned of this, they felt both confused and contented by the knowledge that he had given the throne to have his happiness. Sif perhaps felt the greatest sadness of it all for she was sure now of his choice for the mortal Jane. Though she had never told him how she felt; or still felt toward him; and so Sif resigned herself to still care for him and wait hoping one day he might feel the same for her. But she would always remain a loyal friend and comrade in arms. Now Sif is striving to secure peace and safety for Earth as her beloved home is no longer.