Sif is a friend of Thor's and the Warriors Three. She first appeared in Thor, appeared again in Thor: The Dark World, in two episodes of Agents of S.H.I.E.L.D., and will appear in Thor: Love and Thunder. She is portrayed by Jaimie Alexander.
Thousands of years after the war between Asgard and Jotunheim, Odin would give way as King to his son, Thor (very despite the discontent of Loki). However, the ceremony was interrupted by Frost Giants who wanted to retrieve the Casket of Ancient Winters. This intrusion angered Thor who asked Sif and the Warriors Three to accompany him to Jotunheim and put an end to the cold war between the two races. Sif and others traveled to Jotunheim, even though they thought it was a bad idea. Here, they fought against Jotuns, who quickly surpassed in number and power. Fortunately for them, they were saved by Odin.
She watched as the all-father banished Thor and send him to Earth. Days later and after Odin fall in his sleep, Sif and her friends asked Loki to forgive Thor and allowed him to return from his exile. Loki refused this request since for him that would be the wish of his father. As Loki refused, she and the Warriors three traveled Midgard, even though they knew that it could cause their banishment. Here, they revealed to Thor that his father was not dead, only sleeping, finding that Loki was behind all this.
When the Destroyer arrived, Sif bravely fought against the armor, but she quickly was overpowered by the machine. She had planned to die and have stories told of her sacrifice. But Thor convinced her to live and tell those stories herself. She witnessed how Thor sacrificed himself and so regain his powers. With the Destroyer defeated, Sif and the others return to Asgard.
- “I had things under control!”
Sif traveled with the other Asgardians to Vanaheim to stop the Marauders, who were causing great damage around the Nine Realms. She and the other fiercely fight against Marauders. When Thor arrived, he easily defeated the Marauders. The Marauders then called their best warrior, Korg. Sif allowed Thor fight the monster alone. Thor defeated Korg with one blow and the battle ended. She and the other Asgardians returned to Asgard to celebrate the victory. Tired of the party, Thor decides to leave but he is interrupted by Sif. Sif told Thor that she knew he was visiting Heimdall only to know how Jane was.
After an attack by Malekith and his troops, Sif attended the mass funeral in honor of all the fallen soldiers in battle. The next day, she was summoned by Thor to devise a plan to prevent another attack from Malekith to Asgard. Thor then sent her to liberate Jane from Einherjar Guards and so take Jane to the Dark World and allow Malekith remove the Aether from her. Sif helped to free Jane from the guards. When Thor and Loki were preparing to go to Svartalfheim, she threatened Loki to kill him if it occurred to him to betray Thor.
After finishing the battle against the Dark Elves, Sif and Volstagg went to the Collector's Museum, to hand over the Aether because it is dangerous to have two Infinity Stones in one place. Sif warned Tivan that he must protect the Aether at all costs.
As a result of the attack of the Dark Elves, many prisoners of Asgard escaped including Lorelei, an Asgardian sorceress. Loki disguised as Odin sent Sif to capture Lorelei. Upon reaching Earth, Sif finds Coulson and his team, who decide to help her.
Meanwhile, Sif and Lorelei talking, it was revealed that Lorelei used her powers to seduce the lover of Sif. After many problems, Sif captures Lorelei and return to the dungeons of Asgard. Before leaving, Coulson aks Sif that she did not tell Thor that he is still alive, Sif accepts the request and return to Asgard to confine Lorelei for her crimes.
Sif was sent to Earth by Odin to stop a Kree who had gone to Midgard with unknown reasons. She fought fiercely against the Kree, however Vin-Tak used a device to erase her memory. Later, she met with a group of teenagers, one guy tried to seduce her, causing her to hit him. The police managed to stop her, with the promise that they would bring Kava to her. Coulson and his team arrived to Faro, Portugal; they told her that she and S.H.I.E.L.D. worked together in the past.
Sif, Coulson, and May walked around the place where she and Vin-Tak had fought earlier; Sif said she did not remember anything of what happened. When May asked her about what she remembered of Thor, she could give no account, but admitted that the name alone made her want to smile. Meanwhile, Skye and Bobbi Morse had an encounter with Vin-Tak. That night, they managed to capture him in Chaves. Sif demanded answers from the Kree; Vin-Tak told her that he used Truncheon to erase her memory so she would not interfere with his mission. He then used Truncheon to restore her memory.
Vin-Tak informed the Asgardian that he went to Earth to destroy all Kree experiments, called Inhumans. Suddenly the floor began to tremble, then Sif and Vin-Tak realized that Skye was one of those experiments. Sif pursued Skye and ordered May to hand over Skye. Cornered, Skye decided to shoot herself to prove that she was not a monster. Later, Sif took Vin-Tak and returned him to Hala.
In the fourth episode, after being captured by the Time Variance Authority on Lamentis, Loki is thrown in a Time Cell by Agent Mobius as a punishment, and it is forced to relive a memory of the moment he was beaten by Lady Sif after he cut her hair to play a prank on her. Loki initially thinks he can handle reliving a bad memory, but then he realizes it is also a time loop when he sees Sif appearing again after she just left. Sif hits Loki and tells him that he is pathetic and deserves to be alone for what he did. Loki attempts to stop her, even telling her that she is just a reconstruction of a memory that is being controlled by the TVA. He asks for her help to escape, however, Sif isn't convinced are continues to beat him several times.
She beats Loki to the point that he becomes so defeated and admits to be a horrible person. Loki tries to apologize to Sif, saying that he cut her hair because he thought it would be funny and explains that he is a narcissist that craves attention because he fears being alone. The apology makes Sif pause and she doesn't hit him, but she still states that he deserves to be alone forever. Loki is later removed from the cell to be interrogated by Mobius, but when he refuses to reply the questions, Mobius threats to send him to the Time Cell again. This causes Loki to quickly make up a lie to answer him and avoid having to deal with Sif's fury, but after the interrogation, he is sent to the cell again anyway.
In the third episode, in an alternate timeline set during the events of Thor film, Sif joins the Asgardian army assembled by Loki after knowing that Thor was murdered by an agent of Midgard during his banishment from Asgard. Sif travels to Earth along with Loki and the army, as the God of Mischief wants to avenge his brother's death. They arrive at a base of S.H.I.E.L.D. in New Mexico and meet with Nick Fury. Sif stands beside Loki as he attacks the members of the organization using the Casket of Ancient Winters. Fury attempts to talk to him, saying that destroying Earth won't bring his brother back, and Sif convinces Loki to stop the attack by telling him that his father Odin would want them to hear what Fury has to say.
In the seventh episode, in an alternate timeline set during the events of Thor film, she follows Thor to the penthouse, along with Hogun, Fandral and Volstagg.
The Avengers: Earth's Mightiest Heroes
She was seen leading the Valkyries alongside Brunhilde against a trio of Frost Giants who had besieged both Giant Man and an unconscious Wasp. Sif assisted in several battles during war against Loki, but was defeated. She was later present when Odin took his place on the throne, and cheered for the Avengers alongside her fellow Asgardians.
While Thor stayed in Asgard, Sif overheard how Thor missed Jane Foster, a mortal from Midgard. She interrupted them remarking Thor that he should return to Midgard. Thor went away after Odin arrived and reported that Surtur was missing from Muspelheim and the Warriors Three reported that something was troubling Heimdall, so Thor went to find out the cause.
She was present when Odin summoned Beta Ray Bill who was holding oned Beta R. She attacked without hesitation the Korbonite, but was easily defeated. He was then taken hold by Odin and summoned Thor. Bill explained his backstory and Sif offered to help him to save his people from the Fire Demons and offered her sword after Odin rejected the proposal of Thor giving Mjolnir to him and suggested that they have Eitri, a dwarf, make a hammer for Bill.
After Bill acquired the hammer Stormbreaker, they went to the Skuttlebutt, currently being attacked by the Fire Demons. While Thor and Bill battled the demons, she went into the ship and activated the system of the Skuttlebutt, and ordered the ship to go after the Enchantress who was commanding the Fire Demon army, but was stopped by the ambush of more demons. She went to the place of the sleeping pods where the Korbonites rested and fended off the Fire Demons, until they vanished after Thor and Bill defeated the Enchantress. After they arrived at the Skuttlebutt, she offered to Bill to protect the Korbinites, but Bill refused since he thought her duties lied elsewhere. She bade farewell to Bill and left with Thor apologizing to him for earlier.