Stella is a teenage cystic fibrosis patient who treats her medical regimen with fierce dedication. She creates YouTube videos documenting her illness and is developing a chronic illness management app with medication charts and schedules. Reeling from her parents' recent divorce, she feels immense pressure to survive to protect them from further grief.

Will is a cystic fibrosis patient approaching his eighteenth birthday, an artist who resents the strict confines of hospital life. Diagnosed with a dangerous bacterial infection called B. cepacia, he has lost his place on the lung transplant list. He travels between international hospitals under the direction of his wealthy mother, longing for independence and a chance to experience the outside world.

Abby is Stella's adventurous older sister. She acts as a vital anchor for Stella, providing her with drawings, a stuffed panda named Patches, and stories of outside experiences. Her fearless approach to life contrasts sharply with Stella's medically constrained reality.

Poe is a cystic fibrosis patient and Stella's childhood best friend, having met her during her first stay at Saint Grace's Hospital. He is a gay teenager whose parents were deported to Colombia, leaving him as a ward of the state. He struggles with the financial and emotional burden his illness places on his romantic partners.

Barb is the respiratory therapist at Saint Grace's Hospital. She maintains strict surveillance over the cystic fibrosis patients to prevent cross-infection. Having witnessed past tragedies involving patients who broke isolation rules, she is fiercely protective and uncompromising regarding hospital protocols.

Dr. Hamid is a physician at Saint Grace's Hospital managing the severe cases on the cystic fibrosis ward. She oversees Will's enrollment in a new clinical trial and monitors Stella's declining lung function and subsequent infections.

Stella's mother is grappling with the collapse of her family structure following a recent divorce. She relies intensely on Stella's survival as her remaining anchor, spending late nights researching lung transplants.

Stella's father is navigating life apart from his family after a recent divorce. Stella actively manages his anxiety by hiding the severity of her symptoms and lying about her hospital transportation to spare him additional worry.

Will's mother is a wealthy, single parent who dedicates her resources to extending her son's life. She constantly relocates Will to various international hospitals to enroll him in cutting-edge clinical trials, a strategy that leaves him feeling isolated rather than cared for.

Julie is a pregnant nurse working on the cystic fibrosis ward at Saint Grace's Hospital. She provides daily medical care and friendly interaction for the long-term patients.

Camila is one of Stella's high school friends. She visits Stella at the hospital before embarking on a senior class trip to Cabo that Stella is too sick to attend.

Mya is a high school friend of Stella's who prepares to leave for the senior trip to Cabo. She represents the normal, unrestricted teenage life that Stella is denied.

Jason is Will's best friend from outside the hospital. He visits Will at Saint Grace's and occasionally uses Will's private hospital room for privacy with his girlfriend.

Hope is Jason's girlfriend who accompanies him to visit Will, representing the healthy, unrestricted teenage life that the cystic fibrosis patients observe from afar.

Michael is Poe's ex-boyfriend. He still cares deeply for Poe, though Poe continually pushes him away to avoid burdening him with the reality and expense of cystic fibrosis.