Diamond is an intelligent, overweight Black teenager who is the only person of color in the 1987 town of Swift River. Isolated and deeply affected by the racism in her community, she secretly saves money from her motel job to pay for driver's education. Her primary goal is to escape the claustrophobic town where her father vanished seven years earlier.

Rob, known as Pop, is Diamond's father who disappeared mysteriously when she was a child. Prior to his disappearance, he worked at the local mill until a racist accusation cost him his job. The constant harassment and isolation of being the only Black family in Swift River slowly wore him down, creating severe tension in his household.

Annabelle, or Ma, is Diamond's white mother who struggles to keep her family financially afloat. She works tirelessly and focuses her energy on working through the legal system to have her missing husband declared dead so she can access his life insurance. Her perspective on Swift River differs sharply from her daughter's, causing friction between them.

Lena is a nurse living in Woodville, Georgia, who reaches out to Diamond after years of silence. She operates the family business, Newberry Fine Fabrics, and decides to share family history and heirlooms following the death of her mother. Her letters provide crucial context about the Newberry family's past.

Clara is Diamond's great-aunt and the only Black resident who remained in Swift River after the rest of the community fled racial violence. She works tirelessly alongside the local physician to deliver babies while carefully surviving the town's dangerous sundown laws. Her correspondence captures the systemic prejudice she faces and her determination.

Sweetie is Lena's mother and Clara's younger sister. She acts as the primary recipient of Clara's letters from Swift River, bridging the geographical divide between the separated family members. She also served as a mother figure to young Rob Newberry before he was relocated.

Shelly is a teenager taking driver's education for the third time. She sleeps in her car or outside due to her crowded living situation and dreams of moving to Florida to live with her mother. She forms an unlikely bond with Diamond based on their shared desire to leave town.

Mr. Jimmy is the local driving instructor in Swift River. He uses his position to act inappropriately with teenage students, particularly targeting Shelly during their driving sessions while Diamond watches from the back seat.

Tom is a wealthy former coworker of Rob Newberry who lives in a large house passed down through his family. He holds the power to help Rob find employment but remains dismissive and uses racist nicknames.

Mindy is Tom Campbell's wife. She co-hosts a neighborhood gathering and avoids answering Annabelle's questions when she later searches for her missing husband.

Tommy Jr. is Tom Campbell's son. He pushes Diamond during a game, prompting Rob Newberry to intervene physically.

Jacques is a French-English translator who arrives in Swift River to assist the French-Canadian mill workers. As the only other Black person in the 1915 town, he quickly forms a deep connection with Clara, sharing his language skills and challenging the town's oppressive laws.

The unnamed local physician in 1915 Swift River employs Clara as a midwife and clinic assistant. While he recognizes her immense talent and promises to recommend her for medical school, his support is conditional and self-serving.

Miss Rose is the Doctor's wife in the 1915 timeline. She frequently questions Clara about interracial relationships before ultimately fleeing town to escape her marriage.

Champei is a Cambodian refugee adopted by a local family. As the only other person of color in Diamond's school, they initially bonded, but the stress of Diamond's home life caused a permanent rift between them.

Sylvia is Annabelle's mother and Diamond's grandmother. Though deceased by the 1987 timeline, she is remembered for her practical advice on handling schoolyard bullies.

Tilly is Lena's traveling companion who lives with her in Georgia. She actively encourages Lena to reach out to Diamond and bridge the family divide.

Rick is an older former student of Mr. Jimmy. He joins Shelly and Mr. Jimmy on a disruptive outing, drinking at a mobile home park and making inappropriate advances.