Ahmed is a 14-year-old Syrian refugee who arrives in Brussels after losing his family to war and dangerous border crossings. Seeking a safe place to hide, he takes shelter in the unused wine cellar of an American family's rental home. He honors his Islamic faith and his family's memory by surviving quietly, reading discarded magazines, and tending to sick orchids he finds abandoned in the basement.

Max is a 13-year-old American boy spending a year in Belgium due to his father's job. He feels extremely homesick for Washington, D.C., and struggles to learn French at his new school. Discovering Ahmed hiding in his basement gives Max a secret focus and fundamentally challenges his understanding of the wider world.

Inspector Fontaine is a local Brussels police officer who frequently patrols the Howards' neighborhood. Because the rental house once belonged to his grandfather, he feels a sense of ownership over the property. He views the incoming refugees as a direct threat to European safety and openly expresses hostility toward Muslim immigrants.

Baba is Ahmed's father, who leaves Syria to find a safe home for his son in Europe. He instills a strong sense of Islamic charity and moral duty in Ahmed. He gives Ahmed a family heirloom—an Omega Seamaster watch—during their perilous sea crossing.

Madame Pauline is Max's French tutor. She holds intense anti-immigrant views and frequently complains about refugees altering European society. She takes pride in Belgian history, particularly the local resistance during World War II, but refuses to apply those historical lessons to the modern crisis.

Farah is a Moroccan-Belgian student in Max's sixth-grade class. She recognizes Max's confusion with the French language and patiently helps him with assignments. She faces prejudice in Brussels and knows she must work twice as hard as her peers to prove herself.

Oscar is a boy in Max's class and Scout troop. He bullies Max for his poor French and lack of soccer skills, making Max's transition to the new school difficult. His abrasive behavior masks a deep grief over the loss of his own father.

Claire is Max's older sister. She resents their move to Europe and occasionally acts out by sneaking out to parties. Despite their bickering, she and Max maintain an understanding when it comes to keeping secrets from their parents.

Ibrahim is an Iraqi refugee and a friend of Baba. He promises to look after Ahmed when they first arrive in Brussels. When the government denies his family refugee status, he plans to move to the Molenbeek neighborhood while appealing the decision.

Max's father moves his family from Washington, D.C., to Brussels for a work assignment. He provides Max with basic historical context about the Syrian civil war but insists their family must avoid political involvement due to his job requirements.

Max's mother attempts to manage the household during their stressful transition to European life. She frequently argues with her husband over minor household issues and unknowingly leaves dying orchids in the basement, which later provide comfort to an unexpected guest.

Madame Legrand is a teacher at the School of Happiness in Brussels. She runs a strict classroom but remains attentive to her students' dynamics, pairing struggling newcomers with helpful peers and intervening when conflicts arise.

Ermir is a smuggler operating near the refugee camps in Brussels. He preys on vulnerable people seeking safety and demands exorbitant fees for his services. When refugees cannot pay his sudden price increases, he resorts to violence to extract valuable items.