Sif is a character from the Marvel Cinematic Universe. She appears in Avengers: Infinity War and Avengers: Wrack of Dormammu. She is portrayed by Jaimie Alexander.
Sif appears when Thor and Bruce Banner go back to Earth. She is also seen when Thanos attacks Asgard to get the Space Stone and Soul Stone.
Thor was training with Sif, Fandral, Hogun and Volstagg. Sif was in Thor's Team and the Warriors Three were their opponents. Later, Thor received a message and went away.
Some months later, Thor returnes and he finally says to Sif that he loves her.