In literature, a foil is a character who contrasts with another character, usually the protagonist, to highlight particular qualities or traits. The foil's contrasting traits emphasize the characteristics of the main character. 

An excellent example of a protagonist and foil in a popular movie is:

Protagonist: Harry Potter
Foil: Draco Malfoy (from the Harry Potter series)

Harry Potter is the protagonist, who is brave, loyal, and often acts with a strong sense of justice. Draco Malfoy is his foil. Draco, initially, represents the opposite traits to Harry—he is often arrogant, prejudiced, and tends to act out of self-interest.

Their contrasting characteristics—Harry's moral integrity versus Draco's often antagonistic nature—highlight key aspects of Harry's personality, such as his capacity for empathy, his willingness to fight for the greater good, and his resilience against prejudice. This contrast between the two characters emphasises Harry's positive traits and moral choices throughout the series.

This kind of contrast between the protagonist and foil is a common literary and cinematic technique used to deepen our understanding of the main character.