The protagonist of the novel, based on Plath herself and only thinly disguised. She is extremely bright and talented, and has had her abilities recognized and praised her whole life by others, winning prizes and scholarships. Esther is filled with doubts and contradictions. She’s resentful of the wealth and privilege of others but also sees that wealth and leisure as empty and meaningless. She despises fakeness and small talk but also, to some extent, wants to feel as if she belongs. She struggles with society’s expectations that women stay "pure" for marriage and she chafes against the double standard for men and women where sex is concerned. She is also deeply ambivalent about the idea of becoming a wife and mother. Her struggle between madness and sanity composed the story of the novel.

A practical woman who has had to support her family since her husband, Esther’s father, died, about twelve years previously. She pushes Esther to choose a practical career, like being a secretary, and fails to understand Esther’s intellectual and artistic ambitions. She also takes Esther’s mental problems very personally - she continually asks Esther what she, as a mother, did wrong; her mood throughout the book is generally hurt and bewildered.

A rich southern belle who wins the same magazine contest as Esther. She’s very blonde and very tan, and generally makes Esther feel dowdy and inadequate about her appearance when they go places together. Doreen is very bright but also very snobby; she amuses herself by making cutting remarks about others and tends to judge others based on appearances. But she does seem to admire and care for Esther, who she treats rather tenderly after Esther’s bout of food poisoning, and does try to include Esther in all her social forays.

Also a fellow intern with Esther in New York. Betsy is from Kansas and has very simple ambitions: to marry a farmer and have a big family. Doreen refers to her as "Pollyanna Cowgirl." A decent, sweet person, she offers a marked contrast to the character of Doreen.

The smart, successful fiction editor at the women’s magazine Esther works at. Although Jay Cee is not conventionally attractive (a fact Doreen ridicules her for) Esther admires her and wonders what it would be like to be her. At one point, Esther even explicitly wishes Jay Cee was her mother; Jay Cee does in fact attempt to mentor Esther, but Esther is too unsure of what she wants out of life to take advantage of this opportunity. Jay Cee is both perceptive and demanding - she sees something in Esther which she thinks will rise to the challenge, and singles her out of the group of interns at the magazine.

Esther’s boyfriend for a good part of the story. Esther and Buddy are from the same hometown, although he is a few years older and attends Yale; he is tall, smart, athletic and attractive, and Esther secretly likes him for several years before they actually begin dating. After Yale he goes on to medical school, which seems to fit his temperament perfectly, as he is very rational, determined and precise, and in those ways is very different from Esther. For example, although he has never skied himself, he feels since he has carefully watched others ski, he is qualified to teach Esther to ski. He adores his parents, especially his mother, and buys into the idea of being "pure" in mind and body. When he reveals that he’s already had a love affair with a co-worker, Esther is furious at his hypocrisy. He does seem to love Esther - he begins reading and writing poetry in an attempt to have something in common with her - but he doesn’t understand her artistic and intellectual ambitions any more than her mother does.

The first psychiatrist Esther sees; he’s young, attractive, and in her opinion, very arrogant. He doesn’t seem to ask the right questions and also doesn’t seem truly interested in Esther’s well-being. He recommends the brutal shock treatments Esther undergoes and all the patients in his hospital seem like brainwashed zombies.

Also from Esther’s hometown and a classmate of hers at Smith. She dates Buddy before Esther, and Buddy finds her fun to be around - no-nonsense, fun and athletic. She reappears much later in the book when she is admitted to the same private hospital as Esther. At times she seems to admire and even envy Esther - she’s saved all the newspaper articles about Esther’s disappearance, for example - but sometimes also seems to feel very competitive with her - she brags about getting a letter from Buddy, for example. We only ever see into Joan’s mind through Esther, so it’s difficult to know the exact nature of Joan’s mental troubles. She appears to improve rapidly and is eventually allowed to live "out" of the hospital, although she is still being treated. But towards the end of the book, she abruptly commits suicide, and it’s never entirely clear why.

The psychiatrist who treats Esther at the private hospital paid for by Mrs. Guinea. Esther is surprised to have a woman doctor; Dr. Nolan turns out to be smart, insightful, respectful of Esther and even funny, on occasion. Dismissing Esther’s half-hearted objections about sex before marriage, she provides her with a referral to be fitted for a diaphragm. There is a moment when Esther is afraid Dr. Nolan has betrayed her, when she is again required to undergo shock treatments. But Dr. Nolan promises the procedure will be utterly different from the treatment Esther received from Dr. Gordon (which turns out to be true) and stays with her during the entire treatment.

The various men Esther meets and/or dates. Both Frankie and Marco are dates arranged by Doreen so Esther could accompany Doreen and Lenny. Marco is a nasty womanizer who is obsessed with appearances and tries to rape Esther. Constantin is the translator Esther meets through Mrs. Willard; she very much likes him and attempts to sleep with him but Constantin doesn’t seem interested. Irwin is the young professor who is Esther’s first sexual experience. Not believing she is a virgin, he forcefully penetrates her and causes bleeding serious enough to warrant a trip to the emergency room.

An older, wealthy alumna of Smith College, who endows the scholarship which Esther wins to attend college. Mrs. Guinea is unusual in that she has made all of her own money: she is the writer of a number of best-selling but badly-written novels. She has Esther to lunch when Esther has first begun college and does not mention when Esther commits a terrible social mistake (she drinks the water in a finger bowl). Mrs. Guinea is also the one who provides the money to move Esther to the private hospital which helps to treat her successfully. But she agrees to help only after she’s been reassured Esther’s breakdown has nothing to do with a boy; Mrs. Guinea is clearly a smart, independent woman who, unlike so many of the others who are close to Esther, recognizes both her talent and ambition.