Vix is a shy, intelligent 12-year-old living in Santa Fe, New Mexico, with her financially strapped family. She is drawn into a world of wealth and privilege when she becomes best friends with the captivating Caitlin. Sensible and observant, Vix often acts as the grounded counterpart to Caitlin's wild impulses. She values the educational opportunities the Somers family provides and actively plans for her future.

Caitlin is a wealthy, beautiful, and dramatic girl who invites Vix into her privileged world. Raised with few rules by her bohemian father and jet-setting mother, she is fiercely determined to avoid an ordinary life. She approaches the world with an aggressive curiosity and a desire to accumulate adult experiences. Her glamorous exterior hides a persistent fear of aging and losing her appeal.

Lamb is Caitlin's father, a wealthy former hippie who lives on Martha's Vineyard. He provides a sharp contrast to Vix's strict upbringing, allowing the girls immense freedom and operating with very few rules. His laid-back, passive parenting style stems from his own desire for a peaceful, unconstrained life. He frequently avoids conflict and avoids disciplining his children.

Abby is a confident, organized woman who marries Lamb and brings structure to his chaotic Vineyard home. Driven and maternal, she genuinely cares for Vix and seeks to guide the girls through their teenage years. Her presence often causes friction with the fiercely independent Caitlin, who resents the new boundaries Abby attempts to enforce.

Bru is a local working-class boy on Martha's Vineyard who helps run his uncles' construction business. He becomes a central figure in Vix and Caitlin's summer lives as they transition into their teenage years. Earthy and pragmatic, Bru represents a different side of the island away from the wealthy summer tourists, offering Vix a steady romantic connection.

Nathan is Vix's youngest brother, who has muscular dystrophy. He is a sweet, imaginative boy who longs for independence and relies on Vix for connection to the outside world. He cherishes the letters and small gifts Vix sends him during her summers away, living vicariously through her Vineyard adventures.

Tawny is Vix's mother, a hardworking and often stressed woman who works as an assistant to a wealthy local eccentric. Burdened by financial worries and her husband's unstable employment, she can be strict and emotionally distant. She views the Somers family's wealth with suspicion and worries that Vix is becoming too detached from her roots.

Ed is Vix's father, a quiet and kind man who works multiple jobs, including managing a hotel, to support his family. Unlike his wife, he is supportive of Vix's relationship with the Somers family and wants his daughter to experience the world outside their financially constrained New Mexico life.

Lanie is Vix's younger sister. She feels stuck in her family's low-income reality in New Mexico and frequently expresses resentment toward Vix's glamorous summer life. Her perspective highlights the growing distance between Vix and her biological family as Vix spends more time away from home.

Lewis is Vix's younger brother, part of the bustling, financially strained Leonard household. He represents another piece of the chaotic family life Vix leaves behind each summer when she travels to Martha's Vineyard.

Gus is Daniel Baum's friend from Chicago who spends summers at the Vineyard. Initially part of the antagonistic "Chicago Boys" duo who clash with the girls over shared space, he eventually matures. He develops a deep, enduring affection for Vix as they grow from childhood adversaries into young adults.

Daniel is Abby's son from her first marriage. He brings his friend Gus to the Vineyard, forming a disruptive duo that frequently annoys Vix and Caitlin. He struggles with his parents' divorce and the adjustments required by his new blended family dynamic on the island.

Sharkey is Caitlin's older brother. He is an introverted, highly intelligent teenager who prefers to be by himself or work on computers rather than participate in the house's chaotic social dynamics. Despite his quiet nature and tendency to isolate himself, he remains fiercely protective of his younger sister.

The Countess is an eccentric, wealthy older woman living in New Mexico. She employs Vix's mother, Tawny, and has deep historical ties to Lamb's family from his childhood. She entertains those around her with wild stories of her past, demanding constant attention from her dedicated assistant.

Phoebe is Caitlin and Sharkey's mother, a glamorous woman afflicted with wanderlust. She maintains an amicable relationship with her ex-husband, Lamb, but openly admits she is not a very dedicated mother. She prefers to spend her summers traveling the world and pursuing romantic adventures rather than raising her children.

Trisha is Lamb's former romantic partner who runs a local muffin business on the island. A working-class local, she provides a grounded, unpretentious presence on the Vineyard. She later offers Vix practical advice about handling the wealthy summer crowd.

Von is Bru's older cousin who works at the local fish market. He is the initial subject of Vix and Caitlin's adolescent fascination and their imaginative games. He represents the mysterious allure of older teenage boys to the girls during their early summers on the island.

Maia is Vix's college roommate at Harvard. She is highly analytical, talkative, and prone to health anxieties. Despite her tendency to overthink every situation, she becomes a loyal friend to Vix, offering a grounded perspective that contrasts sharply with Caitlin's dramatic influence.

Paisley is a confident, politically minded college friend of Vix's from the South. She is unapologetic about her desires and ambitions, openly discussing her active dating life and political opinions. She becomes a core part of Vix's adult support system alongside Maia.