A"foil" is a minor character whose attributes contrast and therefore highlight the main character's attributes.
in sections 37 and 43 of "the gift of the magi," jim is shown as a foil for della in that
a. della went from an "ecstatic scream" to "hysterical tears," while jim sat on the couch and smiled.
b. della likes expensive combs, but jim doesn't care for costly things like that.
c. della is remorseful about her short hair, but jim likes it better that way.