Suzy is a twelve-year-old seventh grader in South Grove, Massachusetts. Following the recent death of her former best friend, she chooses to stop speaking, believing that silence eliminates unnecessary noise from the world. A logical thinker with a deep interest in science, she becomes fixated on proving that a rare jellyfish sting caused the drowning. She lives with her mother and struggles to adapt to the social shifts of middle school.

Franny is Suzy's childhood best friend who recently drowned during a summer vacation in Maryland. Once closely bonded with Suzy over their shared elementary school experiences, her transition into middle school brings new interests in makeup, boys, and popularity. Her desire to fit in with a new social circle creates a painful wedge between her and Suzy before her sudden passing.

Meg is Suzy's mother and a busy real estate agent. She tries to support her daughter through the difficult transition of middle school and the sudden loss of Franny. She often feels scattered and relies on practical, though sometimes vague, explanations for life's tragedies, which frustrates her daughter's need for scientific answers.

Jim is Suzy's father, who lives in his own apartment following his divorce from Meg. He maintains a weekly tradition of taking Suzy to dinner at Ming Palace every Saturday night. He is a supportive parent but sometimes misreads his daughter's deep interest in complex scientific topics.

Aaron is Suzy's older brother, who attends a nearby university and coaches soccer. Having successfully survived his own middle school years, he offers a slightly more grounded perspective on adolescence than his parents. He tries to check in on his younger sister during her difficult transition.

Rocco is a graduate student and Aaron's boyfriend. He occasionally visits the Swanson home for family dinners. He is friendly toward Suzy and acknowledges the typical miseries associated with being a seventh grader.

Mrs. Turton is an engaging middle school science teacher who introduces her students to vast concepts like the scale of the universe. She recognizes Suzy's isolation and intelligence, offering her classroom as a quiet, safe space for lunch to escape the chaotic middle school environment.

Justin is a seventh-grade student known for his occasionally clumsy antics. He takes medication for attention deficit hyperactivity disorder, which helps him focus. Despite his reputation, he shows genuine kindness toward his peers and makes an effort to connect with his quiet lab partner.

Dr. Legler is a child psychologist hired by Suzy's parents to help her process grief. Referred to internally as "Dr. Legs," she attempts to draw her young patient out of silence by discussing the basic human need for clear communication.

Aubrey is a popular girl in the middle school hierarchy who focuses heavily on appearances. She represents the social changes that pull young students apart, eventually welcoming a shifting social circle to her cafeteria table while excluding those she deems weird.

Jamie is a real-life Australian scientist who works at the James Cook Center for Biodiversity. After watching a video of his intense reaction to a venomous sting, he becomes the primary focus of an elaborate research quest, representing the ultimate authority on the ocean's dangers.

Dylan is a middle school boy who captures the romantic attention of the popular girls. He engages in cruel behavior to impress his peers, such as harming wildlife during a sixth-grade camping trip, which deeply disturbs those watching.

Sarah is a seventh-grade classmate who gives an engaging presentation about insect-controlling fungi. She occasionally visits the science classroom during lunch and attempts to make friendly overtures to isolated students.

Franny's mother is a grieving parent dealing with the sudden loss of her daughter. She holds positive memories of the girls' childhood friendship and remembers how much her daughter admired the protagonist's independent spirit.