In A Tale of Two Cities, Lucie Manette, albeit underdeveloped, is the heroine, the antithesis of the villainous Madame Defarge; Dr. Manette is the archetype of the doppelganger.

LUCIE MANETTE

A flat character, Lucie Manette is the typical Victorian heroine, swooning, but at the same time representing the virtuous and nurturing woman or relentless sincerity that makes a man a better person.  Although she never comes to life for readers, in A Tale of Two Cities Lucie does inspire love from nearly everyone around her. In addition, Lucie encourages Sydney Carton to recognize his noble potential, and she certainly inspires the devotion that motivates Carton in his final act. In Chapter XIII of Book the Second, she tells Carton,

“No, Mr. Carton. I am sure that the best part of it might still be; I am sure that you might be much, much worthier of yourself.”

Carton, then, reveals to Lucie,

Since I knew you I have been troubled by a remorse that I thought would never reproach me again, and have heard whispers from old voices impelling me upward, that I thought were silent for ever. I have had unformed ideas of striving afresh, beginning anew, shaking off sloth and sensuality, and fighting out the abandoned fight. A dream, all a dream, that ends in nothing, and leaves the sleeper where he lay down, but I wish you to know that you inspired it.”

Later, Carton pledges his devotion,

For you, and for any dear to you, I would do anything. If my career were of that better kind that there was any opportunity or capacity of sacrifice in it, I would embrace any sacrifice for you and for those dear to you. Try to hold me in your mind, at some quiet times, as ardent and sincere in this one thing. The time will come, the time will not be long in coming, when new ties will be formed about you—ties that will bind you yet more tenderly and strongly to the home you so adorn—the dearest ties that will ever grace and gladden you. Oh‚ Miss Manette, when the little picture of a happy father’s face looks up in yours, when you see your own bright beauty springing up anew at your feet, think now and then that there is a man who would give his life to keep a life you love beside you!”

DR. MANETTE

Dr. Manette represents the archetype of doppelganger, the split personality of repressed feelings with the present existence of which Sigmund Freud later wrote. In fact, he character of Dr. Manette has been a precursor of Dr. Jekyll and Mr. Hyde. 

Manette finds it difficult to escape his eighteen-year prison experience, reverting in times of great stress to his shoemaking and his old insanity. However, when he is well, Manette is logical and clearly unbiased.  For, he allows his daughter to marry the son of his enemy, he responds with objectivity when Mr. Lorry talks with him about his "friend," and he defends the innocence of his son-in-law when he is arrested by the Revolutionary tribunal in Paris.

A melodramatic technique, duality was a part of Victorian novels in an attempt to draw complex characters, and Dr. Manette is such a character.

Charles Dickens'A Tale of Two Citiesis rich with character archetypes.  Dr. Manette and Lucy Manette are no exception. 

Dr. Manette is 'The Father-Figure' archetype.  The 'father-figure' archetype is usually quite beloved and occasionally needs protecting, like Belle's father inBeauty in the Beast. Dr. Manette is quite mentally unstable after his incarceration, only remembering his cell block number at times, and in other times, suffers under the delusion that he is a cobbler. 

Lucy Manette represents 'The Innocent' archetypal character.  She is an innocent young woman who must face the evils of the world (in the form of the French Revolution and Madame Defarge).  Lucy is not a hero in the story; her role is largely a passive one, and her delicate nature causes her to swoon on more than one occasion.