Griffin is a seventeen-year-old boy in New York living with obsessive-compulsive disorder. His condition dictates specific physical behaviors, requiring him to stand on the left side of people and fixate on even numbers. He experiences severe grief following the sudden drowning death of his first love, Theo, whom he broke up with shortly before Theo left for college. Griffin refuses to take off a green hoodie Theo gave him, wearing it constantly as he struggles to process the loss.

Theo is an ambitious, intelligent boy with a deep affinity for puzzles, pop culture, and animation. He secures early college admission to a program in California, prompting his move away from New York and the end of his relationship with Griffin. His sudden death from drowning leaves a profound void in the lives of his family and two partners.

Jackson is Theo's college boyfriend from California, currently staying with Theo's family in New York for the funeral. He takes a leave from school to deal with his intense grief and seeks out stories of Theo to cope with the loss. He forms a hesitant connection with Griffin, as they are the only two people who fully understand the specific pain of losing Theo.

Wade is the longtime best friend of both Theo and Griffin, forming the third member of their middle school trio. He closely observes the intimate details of their relationship and often acts as a grounding presence. He attempts to support Griffin through the funeral and its aftermath, despite facing continuous rejection from his grieving friend.

Griffin's mother is a supportive parent who works to help her son process his severe grief and worsening compulsions. She maintains a loving household and tries to intervene by suggesting therapy when Griffin's behavior becomes self-destructive.

Griffin's father is an involved, caring parent who openly supports his son's sexuality, even researching gay relationships to better understand his experiences. Following Theo's death, he grows increasingly concerned about Griffin's mental health and pushes for professional intervention.

Ellen is Theo's mother. She tries to hold her family together following her son's death and encourages open communication among the boys who loved him.

Denise is Theo's younger sister. She is six years old when Theo and Griffin come out to their parents, and she grows up alongside their relationship. She struggles deeply with the grieving process during family gatherings.

Remy is Griffin's cousin who attends the family's holiday gatherings. He lacks empathy for Griffin's situation and expresses hostile, homophobic attitudes regarding Griffin and Theo's relationship.

Anika is one of Jackson's friends from New York. She previously spent time with Jackson and Theo playing role-playing games but feels abandoned since Jackson moved to California and prioritized his romantic relationship.

Veronika is a friend of Jackson's who harbors significant resentment toward him. She feels Jackson abandoned their friend group for Theo and failed to support her during a major personal crisis.