Which of the following describes a character opposite to the main character without being the antagonist?

A foil is a character who contrasts with the main character, often highlighting particular qualities of the protagonist through this comparison. This contrast can underscored differences in personality, values, or choices, allowing the reader to gain a deeper understanding of the main character’s traits and motivations. For instance, if the main character is bold and decisive, a foil might be cautious and indecisive, which serves to emphasize the main character's bravery.

Supporting characters can enhance the story and contribute to its development, but they do not necessarily serve to provide a contrast that sheds light on the protagonist. The term "protagonist" refers specifically to the main character, while "intruder" typically denotes a character who disrupts the status quo. Only the foil serves the purpose of reflecting and emphasizing the characteristics of the main character through their differences.