Sif

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

- Sif

Sif is an Asgardian warrior and friends with Thor. She is the Asgardian Goddess of War.

Captain America & Thor: Avengers
When one of the Warriors Three, Fandral, was sent by Odin into Svartalfheim to spy on the Dark Elves, he was captured and taken to the Citadel of Spires, a tower owned by Alflyse, the queen of the Dark Elves. Thor, Loki, Hogun, Volstagg, and Sif decided to save their friend.

The adventorous Asgardians sneaked into Svartalfheim, and though they were attacked by one giant monstruos creature, they reached the Citadel. While Hogun, Thor, and Volstagg attacked the front door, Loki and Sif went through the caverns beneath the tower. Once inside the caverns, Loki attempted to flirt with Sif, but she rejected him.

When they were attacked by giant bats, Loki realized that they are just an illusion. When they entered the tower, they encountered Malekith, Alflyse's royal mage. Loki told Sif to go ahead while he faced Malekith alone. Sif broke into the royal boudoir of Queen Alflyse, where she found Fandral enjoying the Queen's pleasurable company. They were immediatelly attacked by numerous Elf warriors, but they managed to escape. The battle ended in the Citadel's courtyard where Thor used Mjølnir to defeat Kurse and destroy the tower. With Fandral saved, the Asgardians returned home.

Thor
Sif is the goddess of war and the friend of Thor. Thanks to Thor, who believed in her capabilities, she became one of the most apreciated warriors of Asgard, along with the Warriors Three and the Thunder God himself.

Along with Loki and the Warriors Three, Sif is convinced by Thor to attack Jotunheim, in response of an attempt by some Frost Giants to steal the Casket of Ancient Winters. While in Jotunheim, Sif fights alongside with her friends against an army of Giants led by Laufey himself, and manages to slain many of them, but is forced to flee when they're outnumbered. Eventually, the team is saved by Odin, who intervenes to prevent their killing. Helpless, Sif assists to Thor's banishment on Earth.

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. 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. 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.

Thor: The Dark World Prelude
After Odin used dark energy to send Thor to Earth to retrieve Loki and the Tesseract, Sif informed Heimdall that the Allfather was weak but under Eir's care. She then asked about Thor and Heimdall informed her that Thor survived the journey.

Sif was present when Thor returned to Asgard with Loki and the Tesseract. She was also present when Thor rallied his fellow Asgardians, informing them that they were to protect the Nine Realms from the Marauders.

Thor: The Dark World
Sif is one of Asgard's most formidable warriors. Skilled and fearless, she's a trusted and faithful ally to Thor. During his banishment, Sif saw Loki's treachery and risked all to return Thor to Asgard…but on Earth, she saw that she had lost Thor's heart to another—the human Jane Foster. Now at the end of a long campaign to free the Nine Realms from strife, Sif seeks to rekindle her relationship with Thor.

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 orders. Sif is the first person to suspect Loki of treason following Thor's banishment, showing that she is very observant.

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.

Friends and Allies

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

Enemies

 * Loki
 * The Destroyer
 * Laufey
 * Frost Giants

Appearances/Actresses

 * Marvel Cinematic Universe (2 films)
 * Thor (First appearance) - Jaimie Alexander
 * Thor: The Dark World - Jaimie Alexander
 * Marvel Cinematic Universe (2 comics)
 * Captain America & Thor: Avengers
 * Thor: The Dark World Prelude

Behind the scenes

 * Jaimie Alexander had to exercise for hours in a gym daily in order to be prepared for the role, although this kind of preparation wasn't new to her, who'd been in her high school's wrestling team.

Trivia

 * In the 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.
 * Also in the comics, Heimdall and Sif are brother and sister. This was not explored in the film.