Désirée is a foundling raised by Monsieur and Madame Valmonde. By nature, she is a gentle, loving person well-suited to being a wife and mother. She adores her husband and takes pride in her child. When Armand suggests that she's African American, however, Désirée appears to suffer a kind of breakdown. Rather than live with the shame of being labeled black, she takes the child and walks off into the fields, never to return. This act underscores the essential weakness of her character, which prevents her from standing up for her rights or the rights of her child in the face of false accusations.
A character analysis of Desiree reveals that she is a gentle and loving young woman who feels no shame nor inferiority because she was a foundling abandoned by the road when a toddler, then found, claimed, and adopted by the Valmondes. When grown into young womanhood she is as beautiful and charming as she has always been good natured and sweet of temperament. In addition, she loves easily and without judgementalism for we learn that she gives her heart to the first man who dotes upon her beauty--fortunately, he is a man her parents know well and approve of for her (perhaps wrongly, we find out).
As a young wife, Desiree exerts a peaceful and tranquil influence on the hard-hearted Armand, which induces his mean spirit to exert a gentle understanding hand with his slaves instead of a punishing and severe hand so as to escape being what he later becomes when Desiree's influence is lost to him: "the very spirit of Satan." As a young mother, she is overjoyed with her infant and even more overjoyed to see the pride and joy the infant's father experiences. She reaches a level of happiness that almost frightens her when she realizes the change the infant's presence brings in Armand's feelings and behavior:
"he hasn't punished one of them - not one of them - since baby is born. Even Negrillon, who pretended to have burnt his leg that he might rest from work - he only laughed, and said Negrillon was a great scamp. Oh, mamma, I'm so happy; it frightens me."
When her world tumbles around her and Armand rejects her, she is quick to sense the impending doom coming her way and horrified when Armand finally explains the reason for the growing doom to her:
"Tell me what it means!" she cried despairingly.
"It means," he answered lightly, "that the child is not white; it means that you are not white."
On a negative side, rather than fight back and uphold her right to dignity and her humanity--regardless of evidence of racial mixing (wrongly interpreted evidence)--she instantly and immediately yields to despair and takes herself and her beloved baby to cleanse them both of ignominy, shame, and rejection in the bayou.
A cultural note may be helpful in understanding the falling action of the story. You will notice that the text says:
Desiree had not changed the thin white garment nor the slippers which she wore.
This indicates that the action of walking into the bayou occurred on the same day as the pronouncement that her baby was not white. The reason this can be so is that messages would be sent back and forth between L'Abri and the Valmonde plantation in minutes, as a slave would have been sent as messenger, either walking on "the broad, beaten road which led to the far-off plantation of Valmonde" or on horseback, the latter of which seems most probable. Her mother's response to her despairing letter would have been written immediately while the messenger waited for it before rushing back to L'Abri with Madame Valmonde's reply. Thus, everything would have been accomplished in one afternoon and evening, everything from listlessness to disappearance:
She sat in her room, one hot afternoon, in her peignoir, listlessly drawing through her fingers the strands of her long, silky brown hair that hung about her shoulders. ...
Desiree had not changed the thin white garment nor the slippers which she wore. ...
She disappeared among the reeds and willows that grew thick along the banks of the deep, sluggish bayou; and she did not come back again.