Ty is a teenager and the first child born subsea to pioneers in the Benthic Territory. Living underwater has granted him a special ability known as a "Dark Gift," specifically biosonar, which he desperately conceals to prevent Topsider discrimination against his community. He is fiercely protective of his family and his underwater home, often taking risks to secure their independence from the oppressive Commonwealth government.

Gemma is a teenage Topsider and a ward of the Commonwealth who lives in a crowded boarding house on land. She travels to the ocean floor in search of her missing older brother, hoping he can sign her emancipation forms. Unaccustomed to the expansive and communal lifestyle of the underwater pioneers, she brings an outsider's perspective to the Benthic Territory.

Doc Kuntze is the resident physician assigned to the Benthic Territory by the Commonwealth. He provides medical care to the pioneer families, though Ty harbors intense anxiety around him due to past childhood medical examinations. Doc shares information regarding the local outlaws and their origins at a nearby prison facility.

Shade is the notorious leader of the Seablite Gang, an outlaw faction targeting Commonwealth supply ships. Unlike other gang members, he operates without a mask, relying on his unique physical appearance and glowing skin to intimidate his targets. His sudden attacks on local pioneer homesteads escalate tensions between the settlers and the government.

Richard is Gemma's missing older brother. After spending time in a juvenile delinquent facility for breaking rules to visit his sister, he disappeared into the subsea territories. Gemma carries his photograph and actively searches for him, hoping he will grant her legal independence.

Zoe is Ty's younger sister, a pioneer child who also possesses a secret Dark Gift. She collects sea creatures and keeps them in tanks around her room. Following Ty's example, she conceals her ability from Topsiders and medical professionals to protect her family's standing in the Benthic Territory.

Ma is a scientist and one of the original pioneers who helped establish the Benthic Territory. She values the freedom that the ocean provides her family but worries deeply about the safety of her children, especially regarding the social stigma surrounding Dark Gifts. She frequently debates the safety of remaining underwater as outlaw attacks increase.

Pa is a pioneer scientist and the patriarch of the Townson family. He strongly advocates for the underwater lifestyle, arguing that the cramped and restricted conditions on the surface are far worse for his children than the dangers of the ocean. He is a steadfast member of the pioneer community, working to maintain their agricultural independence.

Hewitt is a pioneer child and Ty's neighbor in the Benthic Territory. Like Ty and Zoe, he possesses a hidden Dark Gift, which allows him to instantly calculate environmental data. He relies on the tight-knit pioneer community when outlaws target his family's homestead and sabotage their power generators.

Shurl is a pioneer neighbor of the Townsons. She works to save her family's livestock and property when their home structure begins to collapse from sabotage, relying on the mutual aid of the Benthic Territory settlers to survive the crisis.

Lars is a Benthic Territory pioneer and Hewitt's father. He is injured during a raid on his homestead, prompting his family to seek immediate help from the Townsons and the broader underwater community to secure their surviving resources.

Jibby is an unmarried pioneer in the Benthic Territory who actively seeks a wife, facing the common problem of Topsider women refusing to move subsea. He is eager to welcome newcomers to the settlement and awkwardly propositions Gemma shortly after meeting her.

Benton is the Commonwealth representative assigned to the Benthic Territory. He delivers government ultimatums, informing the pioneers that the Commonwealth will pull vital resources—including the essential Liquigen—until the settlers capture or kill the Seablite Gang.

Ranger Grimes is a law enforcement officer stationed at the Trade Station. He exhibits strong prejudice against the underwater pioneers, dismissing their concerns and claiming that humans are not meant to live in the ocean. He frequently underestimates the dangers posed by the local outlaws.

Ms. Spinner is the director of the Topsider boarding home where Gemma lives. She treats Gemma more like runaway property than a person, demonstrating clear bias against the Benthic Territory by insisting the subsea environment is unfit for a child.

Mel is a bartender who works in the adult-only establishment at the Trade Station. She helps Ty escape a threatening situation with suspected outlaws, ensuring he leaves the premises safely while enforcing the age restrictions of the bar.

Raj is a Benthic Territory pioneer raising his daughter alone. Like Jibby Groot, he hopes to find a partner, highlighting the social difficulties the settlers face in attracting people from the surface to live in their underwater community.

Eel is a member of the Seablite Gang, a group of outlaws composed of former prisoners. He assists the gang in their operations against Commonwealth targets, utilizing specific skills to disable electronics and secure their escapes.