Abby is Adam's youngest child and an artist who lives and works in her late mother's studio, the Arcadia. She often incorporates found objects from the beach into her art, building tiny structures out of seaweed, driftwood, shells, and other detritus. Having previously experienced a brief brush with artistic fame, she is once again on the verge of success while quietly processing past trauma. She is expecting a child and fiercely values her independence, resisting the patriarchal dynamics of her family.

Adam is an aging, highly respected marine biologist at the Cape Cod Institute for Oceanography. Approaching his seventieth birthday and facing retirement, he chooses to stop taking medication for his bipolar disorder, hoping his unmedicated state will help him unlock the secrets of whale language. He is highly ambitious, deeply focused on his legacy, and often dismissive of the women in his life. He struggles to maintain his authority and grandiosity as his mental state fluctuates.

Ken is Adam's eldest child and a wealthy real estate businessman with aspirations of running for the House of Representatives as a Republican. He constantly seeks his father's approval and feels a deep need to project a perfect image of his family and life. Underneath his polished exterior and political ambition, Ken harbors deep insecurities and maintains a heavily strained dynamic with his sister. He stubbornly protects his financial interests and property, attempting to mask his vulnerability with bravado.

Jenny is Ken's wife, a woman from a wealthy Boston Brahmin family who uses her resources and hosting skills to manage their domestic life. Originally Abby's college roommate at the Rhode Island School of Design, she set aside her own rebellious, artistic side to fulfill the role of a supportive political spouse. She attempts to maintain a pristine public facade while managing marital difficulties and her husband's insecurities. She frequently uses alcohol to cope with the pressure of her expected role.

Steph is a Boston police officer who comes from an Irish Catholic, working-class background. Diagnosed with myotonic dystrophy during her pregnancy, she begins investigating her family history after a confession from her mother. She spends her summer on Cape Cod observing the Gardner family from a distance, carefully weighing how to approach them with her newfound information. Her pragmatic, investigative nature contrasts with the dramatic, artistic sensibilities of the people she is observing.

Tessa and Frannie are Ken and Jenny's twin daughters. While they share a close bond with their mother and aunt Abby, their relationship with their father is often fraught. Tessa, in particular, is outspoken and critical of her father's conservative views and focus on private property, frequently challenging his authority. Both girls represent a younger, more progressive generation that disrupts Ken's ideal image of a traditional family.

David is Abby's childhood friend and a political reporter covering the 2016 Clinton campaign. He shares a long, complicated history with the Gardner family, having been close to Abby since they were children. This historical closeness frequently caused friction and jealousy with Ken. As an adult, his connection to Abby remains a significant factor in both of their lives, complicating his own domestic situation.

Toni is Steph's wife and a third-grade teacher who also practices amateur astrology and reads tarot cards. She acts as a supportive and grounded partner, helping Steph process the emotional complications of her family investigation. Toni frequently consults her tarot cards to provide guidance, warning her wife about the emotional burdens of the people they are observing. Together, they navigate the challenges of raising a newborn son.

George is Ken's therapist, running a practice exclusively for men. He is highly perceptive, aiming to help his clients process feelings of shame and trauma. His relaxed demeanor, marked by unbuttoned shirts and eating seaweed snacks, frequently challenges Ken's defensive walls. He serves as a voice of reason, directly pointing out the harmful effects of toxic masculinity and unresolved family issues.

Emily is Adam's deceased first wife and the mother of Ken and Abby. Before her death shortly after Abby's birth, she built the Arcadia, an art studio in the dunes that remains a highly contested property between her children. Her early passing left a profound emotional scar on the entire family.

Mary Beth is Steph's mother, an Irish Catholic woman from Boston. She carried a heavy secret about her past for decades, which she finally admits after Steph presses her for medical history. Her admission sets off a chain of events that impacts multiple families.

Michael is Steph's father who raised her in their Boston home. Protective and emotionally invested in his daughter, he struggles to understand Steph's desire to look into her past, urging her to focus on the family that raised her. He is deeply devoted to his wife and daughter.

Gretchen is Adam's second wife and Abby and Ken's former stepmother. Though she eventually left the family, she maintained contact with Abby and was one of the few adults to recognize Ken's troubling childhood behavior. She provides an external perspective on the Gardner family dynamics.

Rachel is an interviewer for Art Observer. She visits Cape Cod to profile Abby and her artwork, providing an outside perspective on Abby's talent and the unique environment of the Arcadia. She approaches Abby's creative process with deep curiosity.

Frida is Abby's dog and constant companion. She accompanies the artist during walks on the beach and time spent working in the Arcadia studio. She provides quiet comfort during Abby's isolated hours.

Jonah is Steph and Toni's newborn son. After much debate over traditional versus biblical names, his mothers agree to name him after the Old Testament prophet. His birth triggers significant reflection within his immediate family.

Rebecca is David's wife. She is largely unseen but remains a factor in David's complicated personal life and his ongoing connection to Abby. Her presence informs many of the boundaries David navigates.

Peony is David and Rebecca's daughter. Her presence at a Memorial Day party and Ken's inappropriate remarks about her become a sudden source of intense conflict between Ken and Abby. Her maturation forces the adults around her to confront their own behavior.

Reid is Abby's unborn child. The impending birth influences many of the Gardner family dynamics and Abby's own artistic and personal awakening. He represents a shift toward a new future for the characters.

John Kaufman is a powerful Republican committeeman for the Massachusetts state party. His attendance at family functions represents the high stakes of Ken's political ambitions and the pressure placed on the family to perform perfectly.