Sif is an Asgardian warrior who fought alongside Thor Odinson while on Asgard. She's thought of as the goddess of war in Marvel's version of Norse mythology (being more of an earth goddess in the original Viking pantheon).
Native to Asgard, Sif grew up with Thor Odinson and Loki Laufeyson. When she, Thor, Loki and the Warriors Three incited a battle against several Jotuns, the group nearly caused a war between the Jotuns and the Asgardians. Odin, Thor's father and king of Asgard, found Thor responsible and banished him to Midgard (Earth). After Loki sent the Destroyer to terrorize Earth, Sif helped Thor fight it off.
At some point, Sif had captured and imprisoned the sorceress Lorelei, and shackled her with a collar that prevented her from speaking. During the events of Thor: The Dark World, Lorelei escaped after 600 years of imprisonment, forcing Sif to hunt her down again.
Shortly after S.H.I.E.L.D detects Lorelei's transport to Earth, Sif uses the Bifrost to reach Earth, landing directly in front of a response team made up of Phil Coulson and his team. The group tracks Lorelei to a small bar, where Sif easily dispatches Lorelei's band of seduced bikers.
When looking for Lorelei aboard the Bus, Sif is locked inside the interrogation room, due to Fitz being controlled by Lorelei. The airlock is then opened in the interrogation room, forcing Sif to hold onto the top of the plane as she flies out of the room.
After Coulson and May regain control of the Bus, Sif fights Lorelei, eventually reattaching the collar onto the sorceress, breaking her hold on Ward and Fitz. Afterwards, she takes Lorelei back to her Asgardian prison.
Sif later returns to Earth after Heimdall sees a Kree ship land on Earth and sends Sif to investigate. However, while fighting the Kree warrior, a Kree weapon causes Sif to lose her memory. Coulson eventually finds her and, with the Kree warrior she was fighting, restores her lost memory.
The Kree explains to them what the Diviner truly is and how any Inhumans it creates must be destroyed. It's then revealed that Skye is an Inhuman and Sif demands that Coulson allow her to take Skye back to Asgard with her before she loses control over her powers. After a brief fight through the S.H.I.E.L.D. base, Sif realizes how Skye was prepared to sacrifice herself to protect her friends and trusts Coulson to take care of her.
Lady Sif is a natural born Asgardian, with centuries of honed combat prowess.
- Superhuman Strength: She has the natural strength of a natural Asgardian female, combined with extensive training. She was able to fight the Destroyer and the superhumanly strong Vin-Tak.
- Superhuman Durability: She has the durability of an average Asgardian women. She was able to survive a shotgun blast at close range, without even a scratch.
- Superhuman Agility: She also has the enhanced agility & speed of any Asgardian.
- Superhuman Stamina: Like any Asgardian, she tires much more slowly than the average human being.
- Healing Factor/Regeneration: Like any Asgardian, their natural healing abilities are much faster than a normal human being.
- Superhuman Longevity: She is an extremely long lived, much like any Asgardian.
- Combat Skills: Lorelei has even stated that "the graves of Asgard's enemies are filled with men who underestimated Sif." Despite Lorelei's advanced combat skills, she still considered Sif a dangerous threat. Sif is also noted to be one of the greatest warriors of Asgard.
- Multilingualism: She is capable of speaking english as well as the Kree's native language.
- In the original Viking tales, Sif is famously blonde, with some commentators holding that her 'fair tresses' represent a sheaf of wheat, and thus the bounty of the earth.