What is a secondary character who functions as a contrast to a major character called?
A secondary character who serves to contrast the traits of a major character, often the protagonist, is called a foil. Rosaline, for example, serves as a foil to Juliet. Rosaline is, apparently (for she never appears on stage), emotionally distant and does not return Romeo's affections; in fact, she has sworn to remain chaste and never indulge in fleshly pleasures. Juliet, on the other hand, is very passionate and romantic, and she marries Romeo within twenty four hours of meeting him for the first time.
Tybalt, Juliet's cousin, also serves as a foil for Benvolio, Romeo's cousin. Benvolio, as the root of his name suggests, is good and peace-loving. He is constantly trying to break up fights between the Capulets and Montagues, and he encourages others to use good sense in their dealings. Tybalt, on the other hand, hates peace and seems to thrive on violence. He often instigates fights himself or adds fuel to the disagreements begun by others. Their contrast makes Tybalt seem so much more evil and Benvolio so much more benevolent.