Cadence’s 16-year-old sister, who embraces a nonconformist, detective-like role and has a strong sense of personal destiny linked to visions of U-boats. Her investigative skills challenge age and gender norms.

Cadence’s best friend and an honorary member of the Smith family, who maintains a close bond with them while managing her own personal history and future aspirations.

A German medic who defects from a U-boat and is found by Briar, bringing an element of mystery and tension as he proves himself valuable to the Smith family.

The 2016 narrator who, inspired by the legacy of the Smith women, transforms from feeling lost to embracing her role in the family.

A British officer who transitions from an antagonist to a romantic interest for Cadence, reflecting the enemies-to-lovers trope.

A character whose associations create suspicion and conflict due to his and his family’s German background during WWII.

Cadence and Briar’s brother and a soldier whose fate reflects the broader chaos of war.

The practical and nurturing grandmother who supports Cadence and Briar, providing stability and wisdom.

A wealthy woman whose kindness and connections help Cadence pursue her aspirations in publishing.

The head of the Martha’s Vineyard FBI, presented as symbolizing incompetence, particularly in contrast to Briar’s resourcefulness.