Chet is a ten-year-old boy currently living in Elm Hills, New Jersey, with his uncle. Having spent most of his childhood moving across the country for his father's varied business ventures, he longs for a permanent home and genuine connection. He is helpful and eager to fit in with the local boys, though his transient past makes him deeply insecure about his social standing.

Uncle Jerry is Chet's jovial caregiver and the popular owner of the local diner in Elm Hills. A former baseball star whose career ended due to an injury, he is well-regarded in the community and provides Chet with his first real sense of stability. He frequently dispenses advice to his nephew about adolescence, though he dismisses the initial reports of shark attacks as total nonsense.

Captain Wilson is an elderly former whaling ship captain who resides in Elm Hills. Having survived a terrifying encounter with a white shark during a shipwreck in 1852, he vividly remembers the predator's deadly nature. The townspeople largely dismiss him as a senile old man whose mind is failing him, but his past experiences give him a unique, accurate understanding of oceanic threats.

The Shark is a massive, dirty-gray marine predator with pure white coloring underneath and jagged, needle-sharp teeth. Initially thought to be an impossibility in the fresh waters of Elm Hills, it poses a severe and unexpected threat to the town's residents. It functions as a terrifying force of nature that shatters the local community's false sense of security.

Sid is a local boy in Elm Hills who works at the tile factory and attends school with Chet. He acts as one of the de facto leaders of his friend group and frequently initiates interactions with the new boy in town. Like his friends, he has a penchant for pulling practical jokes, which complicates his budding friendship with Chet.

Dewey is another local boy who works at the tile factory and frequently swims at Matawan Creek. He is part of the close-knit trio of boys who run around Elm Hills looking for amusement. He shares snacks from his mother with the group and partakes in the rough-and-tumble socialization typical of the town's youth.

Monty is a boy in Elm Hills who works at the local tile factory with Sid and Dewey. He is vocal and quick to tease, often contributing to the loud, chaotic dynamic of his friend group. He enthusiastically participates in the local culture of testing boundaries through practical jokes.

Dr. Jay is a local medical professional and a friend of Uncle Jerry. He frequents the diner and discusses town events, noting the rising panic in nearby seaside towns. He enjoys teasing Uncle Jerry about local legends like the Creek Devil, showing a jovial side to his professional demeanor.

Mr. Colton is the owner of the local hardware store in Elm Hills and a regular patron of Uncle Jerry's diner. He is a pragmatic businessman who views the growing panic over sharks and monsters as the result of the oppressive summer heat. He represents the skeptical, grounded mindset of the adult community.

Minnie Marston is a young girl who attends school with Chet in Elm Hills. She represents a small piece of normalcy and social integration for Chet, as he harbors a minor crush on her during his attempts to fit into the town.

Chet's mother currently lives in California with her husband, having finally decided to leave her son in a stable environment with Uncle Jerry. She communicates with Chet via letter, promising him that they have a room waiting for him once her husband's luck turns around.

Chet's father is a restless businessman currently chasing his latest venture in California. His constant pursuit of new opportunities across the country is the primary reason for Chet's transient childhood and deep-seated insecurities about never having a permanent home.