Sara Louise is a thirteen-year-old girl living on Rass Island in the Chesapeake Bay. She is dark-haired, tall, and physically strong, spending her summer days out on a skiff catching crabs to earn money for her family. She harbors deep feelings of inadequacy and resentment because she feels entirely overshadowed by her fragile twin sister. Her internal world is highly romantic and imaginative, contrasting sharply with her tough exterior and the physical labor she performs on the water.

Caroline is Sara Louise's blonde, dainty twin sister. Born small and sickly, she requires extra care as an infant, establishing a family dynamic where she is treated as fragile and precious. She possesses a prodigious talent for singing and piano playing, which earns her expensive mainland lessons and the admiration of the islanders. She is self-assured and charismatic, often oblivious to the deep pain her presence causes her older sister.

Hiram is an elderly man in his seventies who returns to Rass Island after decades of self-imposed exile. He originally fled the community in his youth following a cowardly incident on the water during a storm. Upon his return, he moves into his family's dilapidated waterfront home and quickly proves himself to be a worldly, kind, and grandfatherly figure. He challenges the insular perspectives of the young islanders he befriends.

Susan is the mother of the teenage twins and a former schoolteacher. She originally arrived on Rass Island with idealistic notions of educating local children, eventually settling down with a crab fisherman. She makes constant financial sacrifices to support Caroline's musical education. Though she insists she loves her daughters equally, she struggles to notice Sara Louise's silent emotional turmoil.

Truitt is a hardworking crab fisherman who was born and raised on Rass Island. A veteran of World War I, he returned home and married an outsider schoolteacher. He works long hours on the water to provide for his family, sometimes serenading the oysters with old Methodist hymns. He is a grounding presence in the household but maintains the traditional island view that the water is the domain of men.

Grandma Bradshaw is Truitt's elderly mother and a lifelong resident of Rass Island. She practices a harsh, judgmental version of Methodism, frequently weaponizing scripture against her own family members. Bitter and increasingly paranoid, she directs most of her venom toward her daughter-in-law and her granddaughter Sara Louise.

McCall, known as Call, is a pudgy and bespectacled fourteen-year-old boy. Raised in relative poverty by his mother and grandmother after his father's passing, he serves as Sara Louise's crabbing partner and only real peer connection. He is highly literal and slow to mature, often failing to understand his friend's jokes or her interest in the broader world.

Trudy is an eccentric elderly woman living in a decaying house on Rass Island. She lives alone, surrounded by a large population of feral cats. The other villagers view her with suspicion because she does not attend church services. Unbeknownst to her neighbors, she inherited a massive fortune from her father in her youth but lives as though she has nothing.