Martha is a 12-year-old girl from Wisconsin who travels to Cape Cod to spend the summer with her grandmother. She holds a strong interest in writing and uses fictional stories to process her shifting emotions. Following the sudden death of her classmate Olive, Martha carries a piece of Olive's journal and frequently imagines how the other girl would react to new situations.

Olive is a deceased middle school student who died in a bicycle accident before the summer began. She dreamed of becoming a novelist and seeing the ocean. Her private journal entry reveals she secretly admired Martha, contradicting the quiet, isolated image she projected to her peers at school.

Godbee, whose legal name is Dorothy Boyle, is Martha's aging grandmother who lives in a rustic cottage on the coast. She is open about her physical decline, noting her loss of taste and smell, but retains a sharp mind and a frank communication style. She asks Martha to share one personal detail with her every day of their visit.

Dennis is Martha's father and a former lawyer. He quit his legal job to write a novel, making him the primary daytime caretaker for his young daughter. He often appears stressed by the constant noise of his household and feels anxious about his mother's aging.

Alice is Martha's mother and a broadcaster for Wisconsin Public Radio. She provides financial stability for the family while her husband attempts to write. Her professional, questioning demeanor occasionally annoys Martha, leading to sudden emotional clashes between mother and daughter.

Vince is Martha's 13-year-old brother. He shares a birthday with Martha and maintains a habit of visiting her room for brutally honest conversations before sleep. During the family vacation, he spends most of his time sailing and exploring with a large group of local boys.

Jimmy is the oldest of the five Manning brothers who live near Godbee's cottage. At 14 years old, he aspires to be a famous film director and constantly carries a video camera. He projects a cynical attitude toward nature and families, preferring to record chaotic or vulnerable moments for his documentary project.

Tate is a 13-year-old boy who lives near Godbee's cottage. He stands out from his four boisterous brothers by maintaining a quiet and perceptive demeanor. He interacts politely with Martha when she visits the beach, offering a calmer alternative to his older brother Jimmy.

Lucy is Martha's two-year-old sister. She insists on eating only banana baby food and requires a specific, theatrical kissing routine every morning. Her predictable demands provide a fixed routine for the family during their vacation.

Holly is Martha's best friend from school in Wisconsin. She enjoys discussing clothes and local rumors over the phone. She views the deceased Olive as a strange girl and feels no guilt over her death, arguing that she simply ignored her rather than actively bullying her.

Josh is a middle school student who bullied Olive before her death. He circulates cruel stories about her bicycle accident, claiming she caused it by failing to wear a helmet, which prompts Martha to express intense anger toward him.

Kyle is a middle school student in Wisconsin. Martha previously had a crush on him, a secret that caused her severe public embarrassment when Holly loudly teased her about his presence in the school hallway.

Ms. Driggs is a communication arts teacher at Susan B. Anthony Middle School. She adjusts her lesson plans when she notices her students becoming distressed, demonstrating a perceptive and accommodating teaching style.

John is the landlord of a duplex in Wisconsin. He lives on the ground floor and occasionally watches the residents of the upper unit. He recalls his former tenant Olive as a deeply isolated girl who spent her time writing alone on the front steps.