Charles Dickens makes use of the character of Lucie Manette as a foil to Madame Defarge in order to illuminate the frightening evil of the novel's antagonist, and to further the theme of Good vs. Evil.
The use of foils is a literary technique that can further the development both of theme and of character. In A Tale of Two Cities,Dickens designs his narrative with duality and foils; even the opening sentence starts with a classic parallelism that is a foil to his theme: "It was the best of times, it was the worst of times." Certainly, the duality of Lucie Manette and Madame Defarge also illustrates the theme of Good vs. Evil.
Lucie Manette - The quintessential Victorian heroine, Lucie swoons in men's arms, is often overcome with emotion, and is a delicate creature who needs protection. She is compassionate and loving, embracing her father, recently released from the Bastille, whom she does not know since he has been imprisoned most of her life. She bestows her kindness gratuitously upon even the dissolute as she befriends Sydney Carton. In fact, it is her friendship which raises Carton to a higher level as he finds purpose in his life and spiritual resurrection by sacrificing himself for Charles Darnay in his love for Lucie. Furthermore, Lucie is a devoted wife and mother. When she learns of her husband's imprisonment, she comes with her children and maid to France. Each day she patiently stands for long hours gazing at Charles's prison window in the hope that he will see her and be encouraged.
Thérèse Defarge - Certainly in sharp contrast to Lucie is her foil, Thérèse Defarge. The antithesis of Lucie's motherly love, tenderness, and kindness is Mme. Defarge's dark, sinister, even sadistic nature. Rather than supporting life, she destroys it, joining the Vengeance in cheering the decapitations at the guillotine. Indeed, she is the consummate villain. For instance, Mme. Defarge is intent upon revenge for the deaths of her brother and sister at the hands of the Evremonde brothers, and she registers into her knitting anyone she hates or suspects. She is absolutely resolved in her antipathy for the upper class, or anyone else who does not support the bloodbath of the revolution.
It would be easier for the weakest poltroon that lives, to erase himself from existence, than to erase one letter of his name or crimes from the knitted register of Madame Defarge.
In fact, despite her husband's having been a servant for Dr. Manette, Mme. Defarge knits Lucie's name into her cloth for the simple reason that Lucie is going to marry Charles Darnay. She has mercy for no one. For instance, when her husband makes a plea to his wife to spare the Manettes from death, he alludes to the eighteen years of suffering of Dr. Manette, for whom he was once a servant, and mentions how his daughter Lucie has also suffered. Madame DeFarge reacts only with coldness to his suggestion to desist from her terrible vengeance:
“Then tell Wind and Fire where to stop,” returned madame; “but don’t tell me.”
Well, well,” reasoned Defarge, “but one must stop somewhere. After all, the question is still where?”
“At extermination,” said madame. (Bk. 3, Ch. 12)
Later in the narrative, when Mme. Defarge watches for the man she believes to be Charles Evrémonde (Darnay) to die,
[I]t was nothing to her that an innocent man was to die for the sins of his forefathers; she saw, not him, but them.
After the execution, she then takes her way along the streets to find Lucie Darnay and kill her. When Miss Pross discovers her standing in a room at the Darnay home, she demands what the woman wants. Mme. Defarge looks coldly at her and says, "The wife of Evrémonde; where is she?" Because she has come with a pistol to kill Lucie, Miss Pross wrestles with her until the gun goes off, leaving Thérèse Defarge dead and Miss Pross deaf.