Sif is an Asgardian warrior and friends with Thor. She is the Asgardian Goddess of War. She is one of the most appreciated warriors of Asgard, along with the Warriors Three and the Thunder God himself.
Along with Loki Laufeyson and the Warriors Three, Sif is convinced by Thor Odinson 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 kill many of them, but they are forced to flee when they're outnumbered. Eventually, the team is saved by Odin All-Father, who intervenes to prevent their killing.
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. 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 Asgardian’s, 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 throat 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, but only to sacrifice himself which ends in his rebirth in his divine form. Back in Asgard, Sif brings Heimdall to the Healing Room, while Thor battles Loki.
Along with other Asgardian’s, Sif celebrates Odin's return on the throne and Thor's return in the realm, participating in a banquet.