Emily is a twelve-year-old girl living on a boat with her mother. She feels out of place among her human classmates and struggles with her mother's lifelong refusal to let her swim. Upon secretly entering the ocean at night, she discovers she transforms into a mermaid, a realization that brings her a sudden sense of belonging. Her new identity drives her to learn more about her past and the underwater society hiding nearby.

Shona is a twelve-year-old full mermaid who lives in the underwater city of Shiprock. Dedicated to her schoolwork, she hopes to become a siren so she can protect merfolk from human fishermen. She quickly bonds with Emily over their shared feelings of not quite fitting in with their peers. Eager for connection, Shona shares the customs and history of merfolk society with her new companion.

Emily's Mom lives on a boat named The King of the Sea with her daughter, despite harboring a severe phobia of the water. She claims to have very few memories of her past or Emily's father, which causes tension as Emily grows more curious. She is highly protective of Emily and attempts to shield her from the ocean through strict rules and scheduled hypnotist appointments.

Neptune is the formidable ruler of the merfolk, wielding a trident that grants him control over ocean storms and currents. He governs underwater society with absolute authority and strict traditions. He specifically enforces harsh decrees that forbid any interaction between merfolk and humans, policies that shape the prejudices of his citizens.

Mr. Beeston serves as the local lighthouse keeper near the docks where Emily and her mother live. He is a constant, somewhat overbearing presence in their daily routine. He presents himself as an authority on Emily's absent father, offering harsh opinions that discourage Emily from asking questions about her family history.

Jake Windsnap is Emily's absent father, who disappeared shortly after she was born. According to the local lighthouse keeper, he was an irresponsible man who abandoned the family. However, his name also surfaces in old merfolk history lessons, prompting Emily to seek out the truth about his sudden departure.