In the Marvel Cinematic Universe (MCU), the first Avenger is Steve Rogers, also known as Captain America. In the film “Captain America: The First Avenger,” Steve Rogers undergoes a transformation from a weak and frail individual to a super-soldier with enhanced strength and abilities, thanks to the experimental Super Soldier Serum.
Steve Rogers becomes the first person to be recruited into the Strategic Scientific Reserve (SSR), which later evolves into the organization known as S.H.I.E.L.D. In “The Avengers,” Steve Rogers joins forces with other superheroes, including Iron Man, Thor, Hulk, Black Widow, and Hawkeye, to form the Avengers Initiative, a team of superheroes assembled to defend the Earth from threats.
In terms of the chronological timeline of the Marvel Cinematic Universe, the events of “Captain America: The First Avenger” take place during World War II, making Captain America the first superhero to actively operate under the name “Avenger” in the MCU.