Enzo is a thoughtful, energetic terrier-mix who possesses human-level intelligence but lacks the physical ability to speak. He spends his days watching television, educating himself about the world, and observing the humans around him. He firmly believes in a Mongolian legend suggesting that a prepared dog will return as a human in his next life. He views the world through the lens of race car driving strategies taught to him by his owner.

Denny is a talented race car driver and auto mechanic living in Seattle. He treats Enzo with immense respect, talking to him as an equal and sharing his deep knowledge of track racing. He struggles to balance his demanding, low-paying career on the racing circuit with the growing needs of his family. He maintains a calm, focused demeanor inspired by his driving philosophy, specifically the practice of driving smoothly in the rain.

Eve is an independent woman who marries Denny and changes the dynamic of his household. She comes from a wealthy family but chooses to build a life with the struggling mechanic. Though initially distrustful of Enzo, she forms a strong bond with him during her pregnancy with Zoe. She soon begins to suffer from severe headaches and an undiagnosed sickness that puts an enormous strain on her family.

Zoe is Denny and Eve's daughter. She grows up surrounded by Enzo's protective presence and her parents' loving but increasingly stressed dynamic. She is observant and highly intelligent for her age, often mimicking the emotional climate of her household and playing pretend with her toys.

Trish is Eve's wealthy, critical mother. She lives an affluent lifestyle with her husband, Maxwell, and frequently expresses dissatisfaction with Denny's career and financial status. She exerts a strong influence over Eve and desires to maintain close control over her daughter's health and family affairs.

Maxwell is Eve's father, a prosperous man who views Denny as an inadequate provider for his daughter. He uses his wealth to assert dominance and frequently criticizes Denny's choices. He is deeply protective of Eve and Zoe, though his methods often create tension within the extended family and provoke defensive reactions from Enzo.

Annika is a fifteen-year-old girl connected to Eve's extended family circle. She joins Denny for an extended car ride home from a cabin to beat a winter storm. She exhibits inappropriate boundary-crossing behavior and develops a severe, problematic infatuation with Denny.

Mike is Denny's coworker at the mechanic shop. He steps in to help Denny during difficult moments, picking up supplies for Enzo and offering coverage at work when Denny faces personal emergencies.

Tony works with Denny and Mike at the garage. He visits Denny during times of distress, offering support and advice from a practical, outside perspective regarding Denny's personal struggles.

Mark Fein is a confident, expensive lawyer who accepts vehicle maintenance from Denny in exchange for legal consultation. He displays a clinical detachment regarding family matters and frequently distracts himself by flirting with a local barista during client meetings.

Mr. Lawrence is an attorney who inherits Denny's legal affairs. He lacks the high energy and aggressive enthusiasm of Denny's previous lawyer, approaching his duties with a much more subdued, professional demeanor.

Luca is a powerful representative for Ferrari. He evaluates Denny's driving skills during a track test and sees significant potential in his abilities. He relates to Denny's personal struggles and offers him a unique professional opportunity in the racing world.

Denny's father lives on a farm and harbors a long-standing resentment over Denny's decision to leave home and pursue racing. He maintains a tense, strained relationship with his son, having withheld financial and emotional support for years before finally visiting Denny's apartment.

Denny's mother is a blind woman who visits her estranged son. She speaks only to her husband during the visit, reflecting the deep fracture and lack of communication within Denny's immediate family.