Morgan is a fifteen-year-old girl living on Wilneff Island off the coast of Nova Scotia. She manages the stress of her parents' recent separation by compartmentalizing her emotions and relationships into mental "boxes." She harbors a plan to eventually leave her small town so she can live openly as gay, treating her current life as something she must endure and hide from.

Keltie is a cheerful, impulsive blonde girl who reveals herself to be a selkie—a magical being who shifts between seal and human forms. She possesses a compact, strong build and approaches human customs with guileless curiosity. Unlike Morgan, Keltie embraces her true nature openly, insisting that she remains the same person no matter what form she takes.

Serena is the stylish and outwardly confident member of Morgan's friend group. Her parents own *La Reine de la Mer*, a massive new harbor tour boat, and they frequently use Serena's social life—including her birthday party—as marketing opportunities. She values closeness and communication, becoming increasingly hurt as Morgan shuts her out.

Lizzie is the sweet, empathetic member of Morgan's social circle. She works a summer job as a beach lifeguard and favors the color pink. Highly observant, she is often the first to notice when Morgan acts out of character, mistakenly fearing that Keltie might be stalking her friend.

Jules is the tall, red-haired jokester of the friend group. She dresses in casual, slouchy layers and brings a laid-back energy to the girls' interactions, frequently sending selfies and defusing tension with humor.

Aiden is Morgan's younger brother. Following his parents' separation and his father's move to the city, Aiden channels his grief and fear of abandonment into volatility. He frequently lashes out at Morgan and their mother, though his anger masks a deep desire for his sister's attention and a return to family stability.

Min is Morgan and Aiden's mother. She carries the heavy responsibility of single parenthood in the wake of her recent marital separation. Despite her exhaustion and the tense atmosphere in her home, she consistently tries to reach out to her children, offering them patience and attempting to plan family activities.

Earl is a friendly, thick-glasses-wearing neighbor of the Kwon family on Wilneff Island. He is a gentle presence in the community who surprises Morgan by casually mentioning that he knows Keltie from her visits to his dock.

Val is an island neighbor who uses a wheelchair. She joins the Kwon family for dinner, where she voices her complaints about the disruption caused by the Boisseaus' new commercial tour boat. She provides a sense of community connection for Morgan's family.

A local boy in the friends' social orbit. He serves as the primary subject of Lizzie's romantic interests, prompting teasing from the rest of the group.

A teen boy who attends Serena's birthday celebration on the tour boat as Jules's date, much to Serena's mild annoyance as she wants her core group to stick together.

The hired captain of the Boisseaus' large new tour boat. He is responsible for steering the vessel during its inaugural cruise and interacting with the party guests who wander near the wheelhouse.