Rachel is a complicated and, at times, an unlikeable and unreliable character. In her own perspective, she is a divorcée, deeply depressed over her infertility and a failed marriage. She longs for what she believes to be a happy suburban family life, desperate to matter to others and be helpful. Others, however, see her as a creepy stalker with a potential for violence. When she is drunk, she is self-destructive, yet when she is sober, she is relentless in trying to uncover the truth of the events of that drunken Saturday night, turning her into the engine that moves the novel forward. Rachel is an unconventional detective hunting for clues in her fractured memories. Finally facing the truth of her abusive marriage to Tom, she overcomes her emotional dependence, finally rejects victimization, and resumes control over her life, revealing herself to be the strongest female character in the novel. Despite her flaws, she is guided by a strong moral compass and empathy.
Megan is not at all the model wife and glamorous beauty Rachel imagines her to be. Haunted by her painful past—she is responsible for the accidental death of her baby—she is unable to confide in anyone, condemning herself to a string of superficial relationships that distract her from an overwhelming sense of guilt. Although she loves him, she is cheating on her husband Scott with Tom, mirroring the way Anna and Tom cheated on Rachel. When she finds out that she is pregnant, she is finally willing to leave her past behind, come clean with the men in her life, and move on for the sake of her unborn child. Tragically, the moment she wants to do the right thing, Tom does the wrong thing and kills her to keep their affair a secret, turning her into the victim.
Anna defines herself by her looks and her sexual power over men. She is like Megan in that she had an affair with a married man and like Rachel in that her husband is cheating on her. Yet unlike Megan, who feels guilty over sleeping with a married man, she considered her affair with Tom a turn-on. She believes herself to be far above Rachel, whom she views as a loser, and above Megan, whom she views as baby killer who believes in the facade of a wholesome family life. However, she has no moral compass or empathy for those around her but is instead ruled by self-interest. She turns against her husband not because he is a murderer, but because she realizes that she cannot trust him, revealing herself to be the vixen.
At first glance, Tom seems to be a supportive ex-husband who continues to care for Rachel long after their divorce, an attentive husband to Anna, and an adoring father to their baby girl. As it turns out, he is cheating on Anna the way he cheated on Rachel, this time with Megan. A compulsive liar, he has gaslighted Rachel throughout their marriage, holding her responsible for his violent outbursts. He blames the women in his life for any shortcomings and even blames Rachel for making him so mad that he killed Megan.
Scott is very much in love with his wife, Megan, yet his jealous and impulsive nature makes him snoop in his wife's things, overstepping all privacy boundaries. When jealous, he flies into fits of anger and violence, regretting them afterward. His cycle of violence and remorse is typical of domestic abuse. Like Tom, he is an abusive man, attacking Megan when he finds out about her affair and attacking Rachel when he finds out she was lying to him about her friendship with Megan.