Sif

"I'll die a warrior's death! Stories will be told of this day!"

- Sif

Sif is the Asgardian goddess of war and a good friend of Thor and the Warriors Three. She is also known as Lady Sif.

Early life
"And who proved wrong all who scoffed at the idea that a young maiden could be one of the fiercest warriors this realm has ever known?" "I did!" "True, but I supported you, Sif."

- Thor and Sif

Not much is known about Sif's early life. But, it is known that, thanks to her friend Thor, who believed in her capabilities, she became one of the most appreciated warriors of Asgard, along with the Warriors Three and the Thunder God himself.

Jotunheim
"This isn't like a journey to Earth, where you summon a little lightning and thunder, the mortals worship you as a God! This is Jotunheim." "My father fought his way into Jotunheim. Defeated their armies and took their casket! We would just be looking for answers." "It is forbidden!"

- Fandral, Thor, and Sif

When the Frost Giants of Jotunheim interrupted Thor's coronation ceremony by attempting to steal the Casket of Ancient Winters, Sif, Loki, and the Warriors Three were convinced by Thor to attack Jotunheim, to find those responsible. While in Jotunheim, Sif fought alongside her friends against an army of Giants led by Laufey himself, and managed to slain many of them, but was forced to flee when they were outnumbered. Eventually, the team was saved by Odin himself, who intervened to prevent their killing. Helpless, Sif assists to Thor's banishment on Earth.

Disobeying the King
"We all know what we have to do." "We must go. We must find Thor." "It's treason." "To hell with treason. It's suicide."

- Sif, Hogun, Fandral, and Volstagg

Some days later, Sif persuades the Warriors Three to ask Odin to recall Thor back home, but when they arrive in the throne room, they find Loki sitting as the new ruler of Asgard, as a substitute to Odin who's fallen into Odinsleep. Loki denies their request to call his stepbrother back, but they decide to ignore his order and to go to Midgard anyway.

Battle on Earth
"You know I can't go home. My father is...dead because of me. I must remain in exile." "Thor, your father still lives."

- Thor and Sif

Heimdall agrees with them and opens Bifrost Bridge for them, and the four gods arrive on Earth, in New Mexico. Here, they easily spot Thor, and find out that he's been tricked by Loki's lies. Briefly later, The Destroyer is sent to Earth by Loki to kill Thor and the other Asgardians, and Sif engages it in battle with her friends. While the Warriors Three buy her some time, Sif climbs a rooftop and attacks the Destroyer from above, piercing its troath with her spear. Immediately after, anyway, the Destroyer regenerates itself and almost vaporizes her with his energy blast. Sif flees with her companions, leaving Thor, who claims to have a plan, behind, but only to assist his sacrifice and his rebirth in his divine form.

Return to Asgard
"My Queen, I'm so sorry for your loss. How is he?" "He mourns for his brother. And...he misses her, the mortal."

- Sif and Frigga about Thor

Back in Asgard, Sif brings Heimdall to the Restore Room, while Thor defeats Loki once and for all.

Along with other Asgardians, Sif celebrates Odin's return on the throne and Thor's return in the realm, participating to a gargantuan banquet.

Character traits
Being the only female warrior in Asgard, Sif is very proud and always eager to prove herself. She's the closest of friend with Thor and the Warriors Three, and she's an honorable warrior on the battlefield. She often is independent and uneasy in following the orders.

As an Asgardian, Sif possesses superhuman physical attributes such as strength, stamina, speed and reflexes, and she's an incredibly skilful fighter. She wields a double edged spear, which she uses perfectly, either separate as two blades or a single long sword staff.

Powers
Sif possesses all of the various superhuman attributes common among the Asgardians. Sif possesses superhuman strength, speed, agility, reflexes, durability, and is extremely long-lived.

Superhuman Strength: Like all Asgardians, Sif is superhumanly strong and possesses greater physical strength than the average Asgardian female. She was able to hold her own against many stronger foes.

Superhuman Speed: Sif is capable of running and moving at speeds much greater than that of even the finest human athlete.

Superhuman Stamina: Sif's musculature produces considerably less fatigue toxins during physical activity than the musculature of a human being.

Superhuman Dense Tissue: Sif's bodily tissues are denser than the tissue of a human. As a result, Sif is actually much heavier than she appears. This increased tissue density also contributes somewhat to her physical strength.

Superhuman Durability: Sif's body is much more resistant to physical harm than the body of a human being. She is capable of withstanding great impact forces from Frost Giants and the Destroyer, and exposure to extreme temperatures and pressures of Jotunheim.

Regenerative Healing Factor: Despite her body's resistance, Sif can be injured like any other Asgardian. However, her metabolism enables her to rapidly regenerate damaged bodily tissue with greater speed and efficiency than a human being is capable of. Injuries such as slashes and punctures can fully heal within hours, whereas broken bones can heal within a few days. However, she isn't able to regenerate severed limbs or missing organs. After Sif and the Warriors Three took a severe beating from the Destroyer, she returned to Asgard looking no worse for ware.

Extended Longevity: Like all Asgardians, Sif ages at a rate that is much slower than that of a human being. However, Asgardians aren't completely immune to aging, as she is several hundred years old by human standards.

Abilities
Sif is a formidable combatant and has received extensive training in unarmed combat and swordsmanship. Asgard's greatest warrior Thor comments that Sif proved "all wrong who scoffed at the idea of a young maiden being one of the fiercest warriors."

Friends and Allies

 * Thor
 * Odin
 * Frigga
 * Fandral
 * Volstagg
 * Hogun
 * Heimdall

Enemies

 * Loki
 * The Destroyer
 * Laufey
 * Frost Giants

Video game only

 * Fire Demons

Behind the scenes

 * In the non-canon comic Captain America & Thor: Avengers! Sif helps her friends to free Fandral who is captured by the Dark Elves led by Queen Alflyse.

Trivia

 * In Marvel Comics, Sif is Thor's lover and main love interest, not just a friend or a teammate. Her feelings for Thor are hinted in the end of the movie when she looks at him talking with his father.
 * In Norse mythology, Sif is the wife of Thor.
 * Also in the comics, Heimdall and Sif are brother and sister. This was not explored in the film for some reasons.