Child of a poor artist and a French opera girl, Becky Sharp early learns to shift for herself. Her mother dead, Becky's father with "a great propensity for running into debt, and a partiality for the tavern" brings her up. From her mother she has a knowledge of French from her father the ability to ward off creditors. With this heritage of Bohemian blood, and a clever mind, Becky lives by her wits.

In addition to her mental endowment, Rebecca has physical charm, described by Dr. Squills as "Green eyes, fair skin, pretty figure, famous frontal development." Mrs. Bute Crawley laments Rebecca's physical attraction when she looks at her own dumpy, misshapen, blue-blooded daughters.

Becky has one determination: to carve out a place for herself in Vanity Fair. Although she hasn't blushed naturally since she was eight years old, she can blush at will. She exploits her aloneness and lack of protection. She can cry when she wants to, but the most genuine tears she sheds are those when she has to refuse marriage to the wealthy Sir Pitt Crawley, because she has already married his son, Rawdon.

When she and Rawdon are living on nothing a year, Rebecca deals with the creditors. It is she who starts the rumor that Rawdon has inherited from his rich aunt, and thereby gets out of Paris without paying any debts, since she has ordered a newly decorated apartment against her return. It is she who settles for a percentage of Rawdon's debts in England, so that he may return to London for a fresh start.

Among Rebecca's talents are music, both piano and voice. She can sketch, talk French like a native, dance, act, mimic. Not only her physical charm attracts Lord Steyne, but her wit and mimicry and her ability to get money out of him, even when he realizes she is outwitting him. The more money she wheedles out of him, the more amused he is, until the fatal day when Rawdon walks in on the two of them.

Rebecca's ambition is her outstanding characteristic. She sacrifices husband, child, friends to it; but she enjoys the battle. In a letter to Amelia, after Becky has gone to Queen's Crawley, she says, "At least I shall be amongst gentlefolks — and not with vulgar city people." This jibe refers to both the Sedleys and the Osbornes because George has thwarted her marriage with Joseph Sedley. She continues, "You might lodge all the people in Russell Square in the house, I think, and have space to spare."

Although Rebecca is a merciless social climber, has abandoned her child, whom she hates, has destroyed Rawdon and will destroy Joseph, yet it is she who brings Amelia to her senses, and who realizes that the one true gentleman in the whole of Vanity Fair is Dobbin. After eavesdropping on William's talk with Amelia, Becky says to herself, "What a noble heart that man has, and how shamefully that woman plays with it . . . if I could have had such a husband as that — a man with a heart and brains too! I would not have minded his large feet . . ." Rebecca determines to help William's cause with Amelia. For once she acts unselfishly. When she has destroyed the memory of George for Amelia, she soothes and kisses her, a "rare mark of sympathy with Mrs. Becky." Rebecca also protects Amelia from the two ruffian friends who follow Rebecca and are intent on exploiting Amelia.

Thackeray explains why Becky does what she does: "She was of a wild, roving nature, inherited from father and mother, who were both Bohemians, by taste and circumstance . . . " Becky succeeds in establishing herself in Vanity Fair, at the cost of the lives of two men and the alienation of all her friends and family. She serves as a direct contrast to Amelia.