In Guy de Maupassant's short story "The Necklace," the protagonist is Mathilde Loisel, a young woman who dreams of a more luxurious life than the one she has with her husband. She is dissatisfied with her modest existence and yearns for wealth and glamour.
The antagonist can be viewed as the circumstances and societal expectations that create Mathilde's discontent. Specifically, her own desires for a wealthier lifestyle and her obsession with appearances lead to her downfall. This ultimately results in a conflict that drives the story's plot. While there isn't a singular character as the antagonist, one might also consider the societal pressures and values regarding wealth and status as key adversities in Mathilde's life.