Leo is a 16-year-old junior at Mica Area High School who originally moved from Pennsylvania at age 12. He fits seamlessly into the conformist culture of his school and co-hosts the student television show, Hot Seat. Naturally shy and highly observant behind the lens of a camera, he is fascinated but deeply unsettled by the arrival of a fiercely independent new student.

Stargirl is a formerly homeschooled tenth-grader who immediately disrupts the bland conformity of Mica Area High School. She wears outlandish outfits, carries a pet rat, and serenades students on their birthdays. Secure in her own identity, she demonstrates a boundless, selfless enthusiasm for making others happy and feels a deep connection to the natural world.

Archie is a retired university professor and paleontologist who runs an informal weekend school out of his home, affectionately calling his students the Loyal Order of the Stone Bone. His house is filled with fossils and bones, reflecting his deep connection to history. He serves as a wise, philosophical guide for the local teenagers.

Hillari is the vocal, complaining center of Mica Area High School's social hierarchy. She fiercely protects the school's uniform culture and perceives any deviation as a personal threat. She leads the charge in ostracizing outsiders and demands strict adherence to the status quo.

Kevin is Leo's best friend, having moved to Mica the same year as Leo. Outgoing and ambitious, he serves as the on-camera host for the school's television show, Hot Seat. He dreams of becoming a talk show host and eagerly wants to put the controversial new student on camera for ratings.

Wayne is a handsome but remarkably unengaged student at Mica Area High School. He participates in no clubs, sports, or academic discussions, yet serves as the ultimate role model for the student body's uniform behavior. His only stated aspiration is to become a male model for GQ magazine.

Dori is a ninth-grade student with brown hair who writes poetry. She stands out from the rest of the student body by becoming the first person to show genuine public kindness to the new girl. She prioritizes authentic friendship over social standing.

Cinnamon is Stargirl's pet rat and constant companion. He travels with her to school and helps ease the tension between Stargirl and wary students. He possesses a remarkably calm demeanor and inquisitive nature.

Uncle Pete is Leo's relative in Pennsylvania who owns a distinctive novelty tie. His simple gift sparks Leo's lifelong fascination with porcupine neckties and indirectly sets off a chain of anonymous gifting.

Mr. Robineau is the faculty advisor for the high school television program, Hot Seat. He manages the technical side of the broadcasts and tries to ensure the student jury does not cross the line from tough questions to outright cruelty.

Señor Saguaro is a massive, ancient cactus residing at Archie Brubaker's home. Archie treats the plant as an old friend and wise counselor, frequently pretending to consult the cactus for answers to difficult philosophical questions posed by his students.

Barney is a Paleocene-era rodent skull that Archie keeps in his house. Archie uses the ancient fossil as a teaching tool for the local teenagers, reminding them of the deep history of life on earth and the fleeting nature of their high school concerns.