Alison is a thirteen-year-old competitive swimmer and avid reader living in New York. As an only child, she takes her responsibility of looking after her young cousin Emma seriously. Her natural inquisitiveness drives her to investigate the secrets surrounding her mother and aunt's past at Gull Cottage.

Emma is Dulcie's four-year-old daughter. Living in the city with few friends her own age, she is highly imaginative and intensely lonely. Her desperate desire for companionship makes her extremely vulnerable to negative peer influences when they visit the lake.

A young, blonde girl who frequents the shores of Sycamore Lake. She quickly strikes up an imbalanced and cruel friendship with Emma. She possesses a rebellious, mocking attitude and seems to know unsettling details about the Thornton family's past. Historically, the name Teresa represents a lingering mystery from Claire and Dulcie's childhood, linked to a torn photograph.

Dulcie is Ali's aunt and Claire's older sister. She is a vibrant, bohemian artist who raises Emma as a single mother. She decides to return to the family's abandoned lake cottage to work on a new series of paintings, though the tense environment soon drains her typically energetic personality.

Claire is Ali's mother and Dulcie's younger sister. She is an obsessive gardener who struggles with clinical depression and severe migraines. She is deeply fearful of water and vehemently opposes the idea of Ali spending the summer at Sycamore Lake.

Kathie is a local resident near Sycamore Lake who creates quilted artworks inspired by the water. She provides a warm, maternal presence for Ali during difficult moments at the cottage and possesses deep knowledge about the area's history.

Pete is Ali's father and Claire's husband. He is a university mathematics professor who is logical, patient, and level-headed. He often acts as a stabilizing force in the family, balancing Claire's emotional volatility with his rational approach.

Jeanine is a local resident and a childhood friend of Claire and Dulcie. She serves as a link to the past, holding memories of the summers the sisters spent at Sycamore Lake decades ago.

Erin is a local girl who works at an ice cream shop in Webster's Cove. She accidentally hints at the area's dark history when she expresses surprise that anyone is staying at Gull Cottage.

Linda is Teresa's older sister, who historically found her younger sibling difficult to manage. She remains deeply affected by the unresolved grief of her family's past and demands accountability.

Captain Wahl is a police officer in Webster's Cove who handles the legal and procedural aspects of the mysteries surrounding the lake.