The character of Madame Forestier is crucial to the twist in the ending that Maupassant's tales are so justly famous for. Maupassant ironically uses this tale to comment on the dangers of dreaming about wealth and the kind of dissatisfaction that characters have as a result. This, as the tale shows, leads Mathilde into a poverty far worse than she ever had imagined possible, and shows the dangers of greed and a desire for materialism. Maupassant's "punishment" of Mathilde is demonstrated by the ending, when Madame Forestier reveals to her that the diamonds she had lost were in fact only imitation diamonds, and that Mathilde has impoverished herself for nothing:
Mme. Forestier, much moved, took her by both hands: "Oh, my poor Mathilde. But mine were false. At most they were worth five hundred francs!"
Although, given the presentation of other characters in Maupassant's fiction, we might expect Madame Forestier to keep quiet about the necklace so she could keep the wealth, for this story, at least, the ironic twist makes such a revelation vital. The way that Maupassant describes her as being "much moved" by the story of Mathilde shows that she reacts compulsively, being forced to reveal the truth out of pity and sadness at the needless change that such poverty has caused in Mathilde. In this sense, at least, her character is realistic, though arguably her response is to suit the structure of the story more, and its twist ending, than reflecting reality.