Ava is a preteen girl living at Swamplandia!, an alligator-themed amusement park. In her father's absence, she takes on heavy physical responsibilities, including feeding live chickens to the alligators. She watches her older sister with a mix of fear and curiosity, struggling to process the adult world while living in an isolated swamp environment.

Ossie is Ava's older teenage sister who identifies as a medium. She spends her time communicating with spirits she calls her boyfriends, completely exempt from the chores required to keep Swamplandia! running. Her trances distance her from the physical world and her younger sister.

Timothy is a twelve-year-old boy mourning the disappearance of his younger sister. He and his brother search the waters around their island in a crab sled. Using a pair of magical goggles, Timothy sees the glowing ghosts of sea creatures, an ability that frightens him as much as it aids their search.

Wallow is fourteen years old and serves as the driving force behind the search for his missing sister. Unable to use the goggles himself, he rows the crab sled and pushes his younger brother to scour the coastline. He carries immense guilt, remembering his role in sending his sister down a hill on the day she vanished.

Elijah is a camper at Z. Z.'s Sleep-Away Camp for Disordered Dreamers. He suffers from a condition where he dreams vivid, terrifying historical events as if he lived them. He fears sleeping alone and clings to the shared connection he has with his best friend, who experiences identical historical nightmares.

Oliver is a Junior Astronomer spending the summer at a touristy island with his family. Seeking distance from his twin sister and astronaut father, he falls in with a local bully's gang. He hides his nerdy interests, serving as the secretary for the Comical Ironical Crime Ring as they plan to disrupt a sea turtle hatching.

Raffy is a school bully who creates the Comical Ironical Crime Ring. He devises cruel plans, including using a mentally challenged man in a foil costume to confuse baby sea turtles. He leads through intimidation and views cruelty as amusement.

Jacob is the young son of a human mother and a Minotaur father, traveling across 19th-century America in a wagon train. He fiercely defends his father against the prejudice of the other pioneers. He desperately wants to prove his maturity and adopt his father's bullish strength.

Asterion is a retired rodeo worker who happens to be a Minotaur. Frustrated with his leased farm, he pulls his family's wagon himself toward the promise of green western pastures. His stubbornness drives the family forward even as the journey physically breaks him down and alienates the other travelers.

Velina is a human woman married to a Minotaur. She initially drives the wagon for her family but quickly loses faith in the journey. As conditions worsen, she openly doubts the wisdom of the men leading the wagon train and frequently argues with her husband.

Reggie is a preteen boy whose father works as the maintenance man at the Palace of Artificial Snows. He enjoys skating in quiet circles and views the ice rink as a frozen paradise. He joins his friend on an after-hours spying mission, curious about the secretive behavior of the adults in their community.

Badger is Reggie's determined friend. With an ill mother at home, he focuses his anger on his father, suspecting him of infidelity at the local ice rink. He brings Reggie along to investigate the Blizzard, willing to destroy the event if it means uncovering the truth.

Big Red, legally named Lillith, is a fifth-grade student alienated from her classmates. Uncomfortable in her own skin and dealing with an unsettling home life, she seeks refuge in tight, enclosed spaces. While hiding inside a massive conch shell on the beach, she becomes trapped as the tide comes in.

Barnaby works maintenance at the City of Shells, a job he dislikes after giving up his dream of becoming a forest ranger. Focused on catching the last ferry home, he initially resents having to rescue a trapped child. After falling and breaking his leg inside the shell, he adopts a paternal, protective attitude toward his fellow captive.

Sawtooth is an amputee and a former alligator wrestler living in a refurbished boat at the Out-to-Sea Retirement Community. He is the grandfather of Ava and Ossie. Isolated and bothered by the deep silence of the marina, he pretends to be grumpy while desperately craving human interaction.

Tek is a member of the Waitiki Valley Boys Choir living in a glacier-bound town. He resents his new stepfather and prays for his voice to break so he can stop participating in the town's annual singing Avalanche ritual. A plane crash strands him in the ice, forcing him to face the isolation he previously sought.

Rangi is a mute boy of Moa descent adopted by the local cemetery warden. He chose to stop speaking after his adoptive father shot his pet cinnamon bear. Disconnected from the rest of the choir, he frequently runs away in search of the bear's unmarked grave on the glacier.

Claudette is the human daughter of werewolves, sent to St. Lucy's to learn how to live in human society. She receives her human name on the first day. Choosing to remain middle-of-the-pack, she secretly excels at reading and language while calculating how best to survive the nuns' rigid civilization process without drawing attention.

Jeanette is the oldest of the wolf-girl sisters and the fastest to adapt to human culture. She learns to walk upright and speak before anyone else, drawing the hatred of the pack. Despite her polite exterior, she retains a ruthless instinct for self-preservation.

Mirabella is the youngest of the wolf-girls at St. Lucy's. She entirely fails to adapt to human culture, continuing to run on all fours, howl, and bite. Her refusal to conform isolates her from her sisters, who increasingly view her natural behavior as a threat to their own assimilation.