Final answer:
A secondary character designed to highlight features of the main character by contrast is called a foil.
Step-by-step explanation:
A secondary character who serves as a counterpoint to the main characters is called a foil. This type of character is used by authors to highlight the traits of the main character, usually the protagonist, through contrast, emphasizing their characteristics by being the polar opposite.
For example, the presentation of a determined and decisive foil character in literature might serve to emphasize a protagonist's indecisiveness and lack of motivation. It's important to note that a foil is different from an antagonist, which is a character or force that opposes the protagonist. Foils can be friends, rivals, or any other character that brings out certain qualities in the main character.