Betrayal is a theme introduced at the very beginning of Great Expectations. How does the constant stream of betrayal affect the outcome of the plot, and which characters are most affected by it?

The central betrayal in Great Expectations, as in many of Dickens's novels, is the betrayal of innocent children by adults. The "expectations" of the novel can be interpreted as having a second meaning; as well as the money the title refers to, it also suggests that children can expect love, kindness, honesty, and care from their families. The novel is replete with instances in which adults fail to meet the expectation of proper care for children. We also see here that betrayal exists in cycles: one person's bitterness and disappointment over a betrayal leads that person to betray others in turn. Several characters are involved in betrayals.
Miss Havisham: The great villain of the novel, Compeyson, betrays Miss Havisham by arranging to marry her, defrauding her, and, rather than showing up to the church, sending her a letter jilting her, which she receives after she is already dressed in her wedding gown. She has a mental breakdown and reacts to this betrayal by bringing up Estelle to betray men by breaking their hearts. Pip's love for Estelle in part is impetus for his desire to become a "gentleman."
Arthur: Miss Havisham's half-brother conspires with Compeyson to defraud Miss Havisham.
Magwitch: Magwitch is betrayed multiple times by Compeyson, something that leads to his initial heavy sentence and eventual deportation to New South Wales. There he earns the money that enables Pip to become wealthy. He is also initially emotionally betrayed by Pip, who is disappointed that Magwitch, rather than Miss Havisham, is his benefactor, but eventually, they are reconciled.
Pip: When Pip goes to London and enjoys his newfound wealth, he (temporarily) becomes a snob who betrays his old friends and avoids them; he does not see that true worth is about character—not wealth or class. Eventually, though, he redeems himself as he matures.
Compeyson: Much of the plot is driven by Compeyson's successive betrayals. He first betrays his fiance, Miss Havisham, and then betrays his accomplice Magwich. He is a career forger and con artist whose entire life and career are based on the betrayal of trust.