Don is an Australian genetics professor taking a visiting position at Columbia University in New York City. He prefers rigid schedules and standardized meal plans. He recognizes his neurological wiring differs from others, viewing his lack of a formal autism diagnosis as irrelevant to his daily functioning. He works hard to mimic neurotypical behavior and support his wife. Her unexpected pregnancy triggers his intense fear of becoming an inadequate father.

Rosie is a spontaneous, highly educated PhD student researching environmental risks to early-onset bipolar disorder. She defies Don's need for routine, preferring chaos and surprises in her daily life. As she faces the physical stress of an unplanned pregnancy, she becomes protective of her autonomy and highly skeptical of Don's clinical approach to fatherhood.

Gene is a genetics professor from Melbourne, Australia, who frequently uses evolutionary biology to justify his poor personal choices. He separates from his wife after cheating on her and takes a sabbatical at Columbia University. He considers himself an expert on human behavior, though his own family life actively falls apart as he pursues younger women.

Dave is a mild-mannered entrepreneur working long hours to establish a new refrigeration business. He and his wife are expecting a baby through IVF, which causes him quiet anxiety about bonding with a non-biological child. He serves as a steadying presence for Don, demanding basic promises of safety but otherwise accepting Don's unique traits.

Sonia is an accountant and expectant mother undergoing a challenging IVF pregnancy. She feels frustrated by her husband's intense focus on work, wishing he would show more emotional investment in their impending child. She is fiercely protective of Don, willing to bend rules and deceive authorities to help him pass his social worker evaluations.

George is a wealthy, older British man and the former drummer for a once-famous rock band called the Dead Kings. He owns two four-bedroom apartments, dedicating one entirely to piping cold beer up to his living space. He values precision and expertise, quickly hiring Don to manage his cellar and allowing Don and Rosie to live in the spare apartment.

Lydia is a professional social worker assigned to evaluate Don after a misunderstanding at a local playground. She previously met Don at a group lunch, where his direct questions and choice of unsustainably sourced fish deeply offended her. She holds a firm, early bias against Don, outright telling him he lacks the empathy required to be a father.

Claudia is Gene's estranged wife and a trained psychologist living in Melbourne, Australia. After discovering her husband's infidelity, she demands space, prompting his move to America. She possesses psychological insight into relationship dynamics and begins establishing a new life for herself and her children.

Phil is Rosie's father, living back in Australia. He is a protective figure who carries the heavy burden of a past car accident where he had to choose between saving his wife and saving his daughter. He maintains a quiet but traditional presence in Rosie and Don's marriage from afar.

Eugenie is Gene and Claudia's middle-school-aged daughter. She struggles with severe bullying at her school in Melbourne and feels the impact of her parents' separation. She is highly receptive to direct, practical advice about handling her social problems.

Carl is Gene and Claudia's son. He reacts to his father's infidelity with intense anger and entirely refuses to communicate with him. He relies heavily on his mother and sister as he processes the fracture of his family unit.

Inge is an intelligent and capable PhD assistant assigned to work on research projects at Columbia University. She quickly draws the unwanted romantic attention of Gene. She maintains strict professional boundaries and clearly rejects his persistent advances.

Jerome is a resident in Don and Rosie's first New York apartment building. He has a history of causing friction, once dyeing the couple's laundry purple to save time. He directly misinterprets Don's panic attack in an elevator as physical aggression, leading to an altercation that forces Don to move out.

Seymour is a professional colleague of Don's friends who defends Don's highly logical nature. During a tense lunch, he argues that Don's strict adherence to rules and data makes him exactly the kind of person you would want piloting a plane in an emergency.

Trevor is Don's younger brother. He lives in Don's hometown in Australia along with their mother and father, maintaining a distant but consistent family connection to Don.

Stefan is a PhD colleague in Rosie's academic program. He provides her with necessary academic support that she actively refuses to accept from Don.

The Prince is George's adult son, nicknamed by his father. He struggles severely with drug addiction, a condition George feels deeply responsible for after actively encouraging his son to experiment in the past. He wanders New York City and occasionally seeks out his family.