Subhi is a nine-year-old Rohingya boy who has spent his entire life inside an Australian immigration detention center. To survive the harsh conditions, he cultivates a rich inner life, imagining a "Night Sea" that washes into the camp while everyone sleeps. He longs to hear the traditional stories of his people, which connect him to a heritage he has never physically experienced.

Eli is an older boy in the detention center who acts as a protective brother figure to Subhi. He is the sole survivor of his family's journey to Australia, a tragedy he masks with bravery and humor. Resourceful and defiant, Eli runs a secret trading network within the camp to secure basic necessities for the refugees before being forced into the adult men's tent.

Jimmie is a nine-year-old girl who lives in the impoverished town outside the detention center. She grieves the loss of her mother and feels neglected by her working father and teenage brother. Though she struggles with reading, she fiercely guards her mother's written notebook and possesses a deep curiosity that leads her to investigate the nearby camp.

Queeny is Subhi's teenage older sister. Having experienced life outside the camp before their incarceration, she harbors a growing resentment toward their situation. She focuses on the harsh realities of the camp, losing patience with her brother's fantasies as she tries to procure ways to expose the terrible conditions to the outside world.

Beaver is a cruel and abusive guard at the camp. Missing an eye from a past incident with a refugee, he uses his position of power to terrorize and physically abuse the detainees. He takes particular pleasure in destroying their meager belongings and crushing their spirits through violence.

Maá is Subhi and Queeny's mother. The prolonged trauma of incarceration has led to severe depression, often leaving her unable to eat or get out of bed. She used to sustain her children with rich stories of their Rohingya heritage, but her declining mental health has silenced her storytelling and language.

Harvey is a guard at the detention center, referred to by the refugees as a "Jacket." Unlike the other guards, he shows genuine compassion for the camp's residents, using their actual names instead of their boat identification numbers and bringing small comforts to the children. He represents the moral conflict of working within an oppressive system.

Jonah is Jimmie's sixteen-year-old older brother. He is often absent from the house, preferring to spend his time drinking and hanging out with his friends rather than looking after his younger sister or helping around the home.

Jimmie's father is a working-class widower struggling to support his family. He works long hours and is largely consumed by his own grief over the loss of his wife, leaving him emotionally distant from his daughter.

Ba is Subhi's father, a poet and storyteller who was separated from the family in Burma. Subhi waits endlessly for his return and believes the Night Sea brings him small tokens from his father as a promise of reunion.

Nasir is an elderly refugee who has lived in the camp longer than anyone else. Missing a leg after the guards confiscated his prosthetic, he bonds with Subhi and gifts him a "Pebble of Happy" every year for his birthday.

The Shakespeare Duck is a rubber bath toy resembling the famous playwright. Since Subhi lacks possessions, the duck becomes a prized item and an imaginary confidant who voices Subhi's inner thoughts, doubts, and anxieties.

Anka is a character from Jimmie's family folklore, described as a girl born from an egg. She has blindness but finds her way through the world using echolocation and possesses a beautiful singing voice.

Oto is a boy in Jimmie's mother's story who rescues Anka and eventually falls in love with her, surviving the dangers of wartime and displacement alongside his family.

Mirka is Oto's sister in the notebook story. She wears the bone sparrow necklace and attempts to protect her family from encroaching soldiers, demonstrating the importance of the family heirloom.

Iliya is an old healer in the notebook story. He lives in a mountain cave and attempts to help Oto after the village is attacked by soldiers, linking his own survival to the bone sparrow.

Sarah is an investigator from the outside world who visits the detention center to question the refugees about the unrest and the actions of the guards during the violent clash.

Ratticus is Jimmie's pet rat. He accompanies her on her adventures around the neighborhood and provides her with quiet companionship when she feels lonely.

Pietre is a refugee living in the Alpha section of the camp. He relies on Eli's secret delivery network to obtain basic supplies not provided by the camp administration.