Katherine is an 18-year-old high school senior and tennis enthusiast who volunteers as a candy striper at Overlook Hospital. She approaches her developing relationship with Michael with a mix of strong physical attraction and a desire for emotional readiness. She values close ties and carefully weighs her family's advice against her own growing independence.

Michael is a charming high school senior who shares a birthday month and birthplace with Katherine. He is an avid skier and demonstrates a strong, immediate interest in Katherine. Though he has more romantic experience than she does, he generally respects her pace and tries to build a foundation of friendship and honesty with her.

Erica is Katherine's pragmatic and loyal best friend. Unlike Katherine, Erica views physical intimacy as an experience separate from emotional love and actively looks for opportunities to explore her own sexuality before leaving for college. She is level-headed in a crisis and provides a contrasting perspective to Katherine's more romantic ideals.

Artie is Michael's best friend and a talented high school actor who dreams of attending the American Academy of Dramatic Arts. He appears confident while performing on stage but struggles with self-consciousness and depression in his personal life. He faces pressure from his father regarding his college choices and experiences quiet internal turmoil regarding his identity.

Sybil is Erica's cousin, described as having a genius IQ and extensive sexual experience. She is highly intelligent and unapologetic about her active social and romantic life. She plans to attend a prestigious college and serves as an example of a young woman who embraces her sexuality without shame.

Diana is Katherine's mother, who works as a librarian. She approaches parenting with a blend of modern understanding and maternal caution. She openly discusses her own past dating experiences with Katherine, aiming to guide her daughter toward responsible decision-making rather than enforcing strict prohibitions.

Katherine's father is a pharmacist who shares a love of tennis with his oldest daughter. He is protective and harbors significant anxiety about Katherine growing up too fast. He frequently expresses concerns that her relationship with Michael will lead to her making premature lifelong commitments.

Jamie is Katherine's 12-year-old sister. She is a talented artist who pays close attention to her older sister's social life. She frequently teases Katherine about her relationships and observes the shifting dynamics in the household with youthful curiosity.

Katherine's grandmother is a lawyer who once ran for Congress and acts as a financial supporter of Planned Parenthood in New York City. She is a pragmatic, forward-thinking woman who believes in equipping young people with factual information about sexual health rather than relying on moral panic.

Katherine's grandfather is a New York City lawyer who practices alongside his wife. Following a stroke the previous year, he has taken a more symbolic role at their firm. His vulnerability has helped Katherine overcome her childhood fear of the elderly, inspiring her volunteer work at the hospital.

Sharon is Michael's older sister, an anthropologist for the Museum of Natural History. She owns a vacation home in Vermont and acts as a chaperone for Michael and Katherine's ski trip. She is fiercely protective of her younger brother and worries about him getting his heart broken.

Ike is Sharon's husband. He hosts Michael and Katherine at the Vermont ski house and maintains a relaxed, easygoing presence during their stay.

Tommy is Katherine's ex-boyfriend. Katherine reflects on their past relationship as a negative experience driven primarily by his physical expectations. He serves as a contrast to Michael in Katherine's mind, representing an immature approach to teenage dating.

Theo is a 21-year-old college senior at Northwestern who works as the head tennis instructor at the summer camp where Katherine is employed. He initially strikes Katherine as annoying because he calls her 'Kat', but he proves to be patient with the campers and an excellent tennis player.

Nan is the photography counselor at the New Hampshire summer camp. She quickly becomes good friends with Katherine and forms part of a close-knit trio of counselors along with Theo.

Elizabeth is a high school student who performs in the school play alongside Artie. Her brief interaction with Michael at the New Year's Eve party occasionally triggers Katherine's insecurities as Katherine and Michael's relationship develops.

Fred is a boy who attends Sybil's New Year's Eve party. He converses with Katherine and kisses her at midnight, though Katherine is entirely distracted by her interest in Michael during their interaction.