Ava is a 13-year-old girl living on an island in the Ten Thousand Islands off Florida's Gulf Coast. She idolizes her late mother and trains in alligator wrestling, hoping to take her mother's place as the park's star attraction. She remains deeply loyal to her family and the park, preferring to stay rooted in her isolated home rather than adapt to mainland society.

Osceola is Ava's sixteen-year-old sister who copes with her mother's death by withdrawing into the occult. She discovers a book called *The Spiritist's Telegraph* and becomes fixated on contacting the dead via a homemade Ouija board. She channels her grief into a perceived romance with the ghost of a dredging boat worker, leading her to drift further from reality.

Kiwi is the highly intelligent, autodidactic older brother of the Bigtree family. Frustrated by his father's unrealistic business plans and his family's lack of formal education, he leaves the island for the mainland. He takes a low-paying janitorial job at a rival theme park, the World of Darkness, while secretly saving money for his family and studying for his GED.

The Chief is the patriarch of the Bigtree family and the owner of Swamplandia!. A gregarious showman, he treats his daily life as a performance, outfitting his family in mock-Indigenous apparel to attract tourists. He struggles to parent his children effectively, instead pouring his energy into a misguided modernization plan he calls "Carnival Darwinism."

Hilola is the recently deceased matriarch of the Bigtree clan. She was a renowned alligator wrestler who performed a popular nightly act called "Swimming with the Seths." Her sudden death from cancer deeply traumatizes her husband and three children, serving as the catalyst for the family's unravelling and the park's financial decline.

The Bird Man is an eccentric wanderer who makes a living calling unwanted birds away from properties across Florida. He wears a pungent cape made of bird feathers and possesses extensive knowledge of the Ten Thousand Islands. When Osceola goes missing, he offers his pole boat and tracking skills to help Ava search the treacherous channels of the swamp.

Sawtooth is the former patriarch of the Bigtree family who suffers from progressing dementia. He resides in an assisted living facility located on a permanently anchored boat on the mainland, having been removed from the park after an aggressive biting incident.

Louis is the ghost of a seventeen-year-old boy who died many years ago while working on a dredging boat in the Ten Thousand Islands. According to Osceola, he had a difficult life as an orphan during the Great Depression before dying in a workplace accident. He becomes the central figure of Osceola's romantic and supernatural fixation.

Vijay is a young employee at the World of Darkness theme park on the mainland. He befriends Kiwi despite Kiwi's unusual vocabulary and stilted mannerisms. He offers Kiwi practical advice about mainland living, helps him navigate social situations, and gives him rides to his evening GED classes.

Carl is a manager at the World of Darkness theme park. Despite holding a master's degree, he works at the struggling amusement park and spends his time reading fantasy novels featuring scantily clad women on the covers. He is initially dismissive of Kiwi's ambitions but eventually facilitates his movement through the park's ranks.

Whip Jeters is a local park ranger who patrols the Ten Thousand Islands. He is familiar with the Bigtree family and expresses condolences over Hilola's passing. He keeps a watchful eye on the local waterways and exhibits concern when he finds young Ava navigating far from home with a strange man.

Emily is a young tourist visiting the World of Darkness theme park. When her hair gets caught in a pool drain, she requires immediate rescue, an event that significantly alters Kiwi's standing among his mainland peers and launches him into local media.

Dennis is a flight instructor working for the World of Darkness's new aviation attraction. He trains new pilots to fly over the Everglades and is present during a crucial, unexpected detour during one of Kiwi's training flights.

Mama Weeds is a legendary swamp woman of the Ten Thousand Islands who purportedly lived deep in the backwaters working as a laundress. According to local lore, she was murdered by men who objected to her independence, and her ghost is said to haunt the region where she died.

Yvans is a young worker at the mainland amusement park where Kiwi finds employment. He operates within the established social hierarchy of the park's staff, representing the mainstream teenage culture that Kiwi initially struggles to understand.

Leo is a young employee at the World of Darkness. He possesses a fake ID and takes part in the mainland recreational activities that expose Kiwi to life outside the swamps and Swamplandia!.