Foil
A character whose contrast with another highlights that other character’s qualities.
A foil exists largely to set off another character by difference, throwing key traits into relief. Writers use foils to clarify values, sharpen conflict, and underscore theme. The contrast can be in temperament, morality, or circumstance.
Example
Laertes acts as a foil to Hamlet: both lose their fathers, but Laertes’ swift, headlong revenge throws Hamlet’s hesitation and brooding into sharp relief.
Hamlet · William Shakespeare
See it in action
Analyses on StoryBites that use foil: