Charlie is a 12-year-old boy with autism who relies on consistency and routine to feel safe in his Southern California home. Following his mother's death and his father's recent injury in Afghanistan, he uses his deep interest in ornithology to manage an unpredictable world. Washing his hands repeatedly in hot water and eating safe foods like chicken nuggets offer him comfort during stressful times. When forced to take a cross-country RV trip to reach his father's new hospital, he faces his intense aversion to travel and unpredictable environments.

Ludmila is a young woman with bright pink spiky hair and a distinctive heart tattoo bearing the name Amar. She initially appears as a mysterious presence visiting Robert's hospital room before stepping in as an emergency guardian for his children at Gram's request. As a war orphan who survived the Bosnian siege of Sarajevo in the 1990s, she carries profound grief over the loss of her family. She drives a tiny, cluttered car before upgrading the group to a borrowed RV for their journey to Virginia.

Robert is an English teacher and part-time journalist who raises four children as a single father in Southern California. He provides a calming presence in his home and actively supports Charlie's interest in birds by taking him on canyon walks. After suffering a severe head injury from a bomb explosion while reporting on soldiers in Afghanistan, his sudden hospitalization upends his family's predictable routine.

Davis is a 15-year-old high school student and the eldest sibling in her family. She frequently helps Charlie interpret social cues and human emotions, serving as a vital guide for his understanding of the world. While she often focuses on her own teenage interests and her boyfriend Jonathan, she displays fierce protectiveness over her siblings when she suspects their temporary guardian might be untrustworthy.

Gram is the pragmatic grandmother who steps in to manage the household following Robert's injury in Afghanistan. She exhibits a strict approach to parenting that initially intimidates Charlie but demonstrates her firm commitment to her family's physical well-being. She travels to the East Coast to coordinate her son's specialized neurology care, leaving her grandchildren behind until an emergency forces her to arrange alternative supervision.

Joel and Jake are ten-year-old twin brothers who bring constant energy and noise to their household. Their rowdiness and lack of organization often disrupt the quiet environment their older brother Charlie prefers. Despite occasionally teasing Charlie about his particular habits, they participate enthusiastically in the family's cross-country adventure and quickly adopt a stray three-legged dog they find in Las Vegas.

Amar is Ludmila's brother who protected her during their childhood survival of the siege of Sarajevo. He joined the military to find purpose, a decision that created tension in his relationship with his sister. He served in Afghanistan, where he crossed paths with Robert during the journalist's reporting assignment.

Tiberius is a three-legged stray dog discovered by the twins on the side of the road in Las Vegas. Initially smelly and overwhelming to Charlie, the canine quickly forms a strong attachment to the boy. The dog travels with the family in the RV, offering silent comfort and companionship during moments of high stress.

Dr. Tiberius Shaw is an elusive ornithologist and author whose work serves as a guiding text for Charlie. His writings draw connections between avian habits and human behavior, offering a framework for understanding social dynamics. He is believed to reside near Sanctuary Marsh in Virginia.

Jonathan Dylan Daniels is Davis's teenage boyfriend who spends time at the family's house playing video games and eating junk food. He agrees to drive the siblings out of California in the middle of the night but proves to be a reckless driver. He directs unkind comments toward Charlie and quickly abandons the group after a minor car accident in Las Vegas.

Dr. Joan is an astronomer running an infrared observatory on a mountain top in Wyoming. She previously served as one of Ludmila's foster mothers after Ludmila arrived in the United States. She generously provides the family with her RV, Old Bessie, for the remainder of their cross-country trip and shows Charlie the night sky through her telescope.

Mariana is an old friend of Ludmila's from Sarajevo who currently lives in Chicago. She coordinates the family's visit to the Field Museum and offers them a place to sleep. She notices Charlie's anxieties about hand-washing and offers him gentle encouragement based on her own past fears of driving.

Helen is a worker at the Field Museum in Chicago and a colleague of Mariana. She takes time out of her schedule to guide visitors through the Hall of Birds and the bird lab. She enthusiastically shares facts about the museum's rare avian specimens, helping Charlie check items off his list.

Dr. Spielman is the neurology expert overseeing Robert's medical care at the hospital in Virginia. He communicates updates regarding complex surgeries and recovery progress to Gram as the family awaits news in the hospital waiting room.

Rodney is a park ranger working at the Sanctuary Marsh in Virginia. He encounters a lost and overwhelmed visitor near the public restrooms. He patiently listens to Charlie's story before offering practical assistance in contacting the boy's family.

Anna is an Amish woman living in Pennsylvania. She invites the traveling family to camp out on her farm for the night. She demonstrates concern for Charlie's safety when hunters discharge a firearm near the property in the dark.

Mr. McGinty is a historical reenactor working at a tavern in Colonial Williamsburg. He stays firmly in character and loudly accuses a confused Charlie of being a misbehaving apprentice, creating an uncomfortable public scene that Charlie initially takes literally.