Essentially a passive character, Charles Darnay is the double of Sydney Carton, who rescues him from prison twice. He is a young gentleman of twenty-five when the novel begins,
...well-grown and well-looking, with a sunburnt cheedk and a dark eye.....He was plainly dressed in black, or very dark grey, and his hair, which was long and dark, was gathered in a ribbon at the back of his neck: more to be out of his way than ornament.....the paleness which his situation engendered came through the brown upon his cheek, showing the soul to be stronger than the sun. He was otherwise quite self-possessed....(Bk.2, Ch.2)
When Darnay is brought to his first trial on a charge of treason, over his head is a mirror which later serves to help create doubt in the jurors' minds as to whether or not he has committed the crime of which he is accused. For, Sydney Carton has noticed, after having closely examined Darnay, that they possess a strong resemblance to one another. And, C.J. Stryver manages his release based upon this doubt.
With his passivity and penchant for imprisonment, Darnay is perfect for Lucie as an outlet for her compassion. When Carton realizes that Lucie has become so found of Darnay, he becomes immediately jealous, recognizing in Darnay what he "has fallen away from....You hate the fellow!"
Once Darnay has become the love of Lucie, Sydney Carton pledges his devotion as well, and Darnay continues as Carton's mirror image. Darnay also acts as a connecting character of the two cities as, having lived in England with Lucie, he is then drawn to the "Lodestone Rock" and returns to France, this time to rescue Gabelle the former tax collector on the Evremonde estate. In addition, his revelation of his family name of Evremonde connects him with Dr. Manette's imprisonment. But, again, Darnay cannot save himself; Dr. Manette's powerful testimony sets him free, however, as the physician is recognized as a hero, having been "the Bastille captive."
After Darnay's second arrest, Dr. Manette is unable to save him. It is then that Sydney Carton's act of sacrifice to take his place again presents the motif of doubles as Carton gains access to the prison where Darnay is. Then, Carton becomes the sacrificial victim for the phlegmatic and passive Darnay who is slipped from the prison to escape France with his family members. Drugged as he is removed from the prison, Darnay cannot attempt to thank Carton for the tremendous sacrfice which he has made by sending someone to carry this message to him.
Despite his passivity, Darnay's character connects the actions of the novels in both London and in Paris. His resemblance to Sydney Carton as his double allows Darnay to act as a catalyst for Carton's noble deeds toward Lucie, and to commit his heroic act to save the Darnay family from fate and unhappiness.