Ross is a seventh-grade boy diagnosed with a rare eye cancer. He is highly self-conscious, using his secret comic strip character "Batpig" to process his fears and frustrations. He relies heavily on his best friend Abby to handle social difficulties and uses drawing as his primary emotional outlet before discovering a passion for playing guitar.

Abby is Ross's flamboyant best friend who intentionally wears eccentric clothes and lets her curly red hair go wild. She plays the viola and tries to shield Ross from the unwanted attention his illness attracts. Her family plans to move to Minnesota, causing her significant anxiety about making new friends.

Jimmy is an imposing classmate who chews massive wads of Big League Chew and spits into a jar he carries with him. He projects a hostile exterior and frequently clashes with Ross. He has a difficult home life and lives in a rundown neighborhood, finding a productive outlet in playing the drums.

Ross's father is a busy trial lawyer who cares deeply for his son. He struggles with the memories of losing his first wife to cancer while trying to remain strong for Ross. He encourages Ross to share his emotional burdens rather than bottling them up, drawing on his own past grief.

Linda is Ross's stepmother who tries to provide stability and comfort during his cancer treatments. She frequently consumes Starbucks green tea lattes and maintains a respectful attitude toward the memory of Ross's biological mother. She offers practical, grounded advice when Ross spirals into panic.

Frank is a radiation tech who plays in a band called Ripe Sponge. He introduces Ross to hard rock and punk music to help him cope with the grueling treatments. He lives with his bandmate Denny in a slightly worn house filled with vintage records and cassette tapes, offering his basement as a safe space for Ross to practice.

Jerry is an older man receiving proton radiotherapy for testicular cancer. He affectionately nicknames Ross "Dime Slot" after seeing the radiation marker scar on the boy's forehead. He shares wisdom from his own life, expressing regret over giving up his dream of playing the trumpet professionally to pursue a "normal" life.

Sarah is a popular seventh-grade girl whom Ross has a crush on. She organizes the school's Christmas talent show and maintains a pristine, friendly exterior that enchants Ross. She is frequently seen navigating the halls with her group of friends.

Isaac has been part of Ross and Abby's friend trio since the fourth grade, famously sealing their bond with the "Great Oreo Pact" at Lake Monroe. Following Ross's cancer diagnosis, Isaac withdraws completely from the friendship because he feels helpless and frightened, opting to hang out with a new crowd instead.

Denny is the drummer for the rock band Ripe Sponge and Frank's housemate. He frequently corrects his cousin Jimmy's bad habits and encourages Jimmy to channel his energy into playing music with Ross.

Callie works as a radiation technician alongside Frank. She assists with the daunting medical machinery and helps guide Ross through his daily proton therapy sessions, maintaining a professional but kind demeanor.

Dr. Throckton is a proton radiotherapy specialist. The Maloy family refers to him as "the Man with All the Answers" because he proposes a method to save Ross's eye using targeted radiation instead of a complete surgical removal.

Dr. Sheffler is the doctor who initially discovers the rare, aggressive tumor above Ross's right eye. He delivers the difficult diagnosis directly to the family, admitting he has never seen this specific type of cancer in his career.

Dr. Inzer is a surgeon who bluntly recommends removing Ross's entire eye and eye socket to treat the rare cancer. She focuses purely on saving Ross's life over preserving his vision or facial structure.

Ms. Bayer is a language arts teacher who gives Ross unexpected special treatment following his diagnosis. She later reports a classroom altercation between Ross and Jimmy to the principal.

Principal Kingsley oversees the middle school. He addresses disciplinary issues but focuses more on resolving the underlying causes of student distress—like online bullying—rather than strictly punishing them for acting out.

Denise is a classmate who openly displays disgust when Ross experiences radiation side effects at school. She assists with organizing and running the school's Christmas talent show.

Ross's mother died of breast cancer when he was four years old. She was a talented illustrator, and Ross uses her old sketchbook cover to hold his own secret drawings. He frequently looks at pictures of her to remember her face.

Chris is a member of Isaac's new social circle at school after Isaac stops speaking to Ross and Abby.

Marilyn is Jerry's wife who cares for him as he undergoes treatment for testicular cancer and subsequently battles severe pneumonia.

Lisa is Abby's viola teacher who supports her musical development and eventually attends gatherings in Frank's basement to offer encouragement.

April is Abby's cousin who lives in Minnesota. She attends the school Abby will soon transfer to, causing Abby anxiety about fitting in.