What was the flat character in the movie Captain America?

In "Captain America: The First Avenger," a flat character is Agent Carter, portrayed by Hayley Atwell. While she serves as a strong and capable ally to Steve Rogers, her character primarily fulfills the role of a love interest and does not undergo significant development throughout the film. Her traits, such as bravery and loyalty, are consistent and serve to support the protagonist's journey rather than evolve in complexity.