Felix is a young German immigrant who stows away on a ship to America in 1845. He lives with his aunt and uncle while working as a runner to save money to bring the rest of his family overseas. Fast and courageous, he loves the developing game of baseball and readily volunteers to help put out a dangerous city fire.

Louis is a young Union soldier who lies about his age to fight in the Civil War in his father's place. He carries a baseball his father made, which brings him comfort during the fighting. He encounters injured soldiers on the battlefield and makes difficult moral choices regarding enemy troops.

Arnold is a young boy in the late 19th century who idolizes the famous baseball player King Kelly. He highly values a baseball bat his father brought home from the Civil War. He attempts to help his failing idol return to professional baseball, leading to a harsh lesson about trust.

Walter is a youth who faces discrimination because his family's surname sounds Jewish. He advocates for a talented pitcher to join a white baseball team by suggesting the player emphasize his Comanche heritage to bypass racial barriers. He struggles to understand the deep-seated prejudices around him.

Frankie is a clever young woman in the 1920s who works as a numbers runner for a local gambling ring. She possesses strong mathematical skills and views numbers as the only true thing in the world, a belief that shifts when she learns how gambling systems operate.

Kat is a professional baseball player in the All-American Girls Professional Ball League during World War II. She struggles with intense anxiety regarding her father's military service, developing compulsions like touching wood and saving gum wrappers to cope with her fear.

Jimmy is a schoolboy growing up during the Cold War. He faces relentless bullying from a classmate and initially tries to hide to protect himself. After learning about nuclear deterrents in school, he applies the concept to his personal conflict.

Michael is a youth baseball pitcher in the 1980s who finds himself in the middle of a perfect game. He struggles with intense perfectionism and fears throwing his curveball because he knows the pitch is flawed, causing him extreme stress on the mound.

Snider is a modern-day teenager living with his uncle after his family's home burns down. Frustrated by his displacement and injury, he finds focus when his uncle tasks him with researching a box of antique baseball heirlooms passed down through his family.

Uncle Albert is a hardworking cutter in 1840s New York. He takes in his nephew Felix and works tirelessly to support his family. He considers changing his German surname to secure better employment but ultimately retains his family name.

Aunt Jenell is Felix's aunt living in New York. She provides a home for her stowaway nephew and supports her husband, Albert, as they try to make a living in America.

Herr Newman is an immigrant living in a windowless tenement in 1845 New York. He works long hours sewing and provides Felix with delivery jobs.

Alexander is an early pioneer of baseball and a volunteer firefighter. He allows Felix to assist the fire brigade during a massive city fire and views baseball as a reflection of the American spirit.

Stuart is a Union soldier serving alongside Louis during the Civil War. He is frequently scared of battle and relies on Louis for companionship and reassurance during their deployment.

Jeremiah is a Confederate soldier blinded during a battle. He depends completely on Louis for survival and bonds with the Union soldier over their shared knowledge and love of baseball.

King Kelly is a formerly great baseball star who now struggles with severe alcoholism and unemployment. He maintains a facade of wealth and success by paying a valet, but in reality, he faces financial ruin and a declining career.

Hiroshi is King Kelly's valet who struggles to manage the fading baseball star's drunken behavior. He quits his position because Kelly can no longer afford to pay his wages.

Donovan is the manager of the Brooklyn Superbas baseball team. He is initially reluctant to let Cyclone Joe Williams try out and ultimately refuses to add him to the team due to racial prejudice.

Cyclone Joe is a highly talented pitcher of Black and Comanche descent. Because of systemic racism, he cannot play in the major leagues and must perform in exhibition games. He faces severe discrimination even when given a tryout for a white team.

Mrs. Radowski is a neighbor who regularly places bets with Frankie. She admits that even if she won a large sum of money, she would likely continue gambling.

Mickey is a man involved in the illegal numbers game who recognizes Frankie's mathematical talents. He oversees the payouts and becomes angry when the system is manipulated against him.

Mr. Jerome is a man working alongside Mickey Fist in the illegal gambling ring. He processes the daily numbers that runners like Frankie bring in.

John is a sports reporter who writes his articles before games even begin. He challenges Frankie's belief in the absolute truth of numbers by explaining how statistics and gambling systems contain flaws and manipulation.

Connie is the talented star pitcher for the Grand Rapids Chicks. Initially distant, she becomes Kat's roommate and supportive friend, helping her adjust to the league and cope with her anxieties.

Pepper is a baseball player from California who bonds with Kat. She explains the financial realities of professional softball out west and guides Kat during a late-night outing.

Ms. Hunter is the strict chaperone for the Grand Rapids Chicks baseball team. She enforces the league's rules regarding feminine appearance, insisting the players wear skirts and maintain proper grooming.

Eric is Jimmy's aggressive classmate who constantly threatens to beat him up. Despite projecting a tough exterior and claiming to be unafraid of bombs during drills, he harbors a deep panic regarding Cold War threats.

Ralph is Jimmy's school friend who attempts to help him navigate the school's exits to avoid Eric's physical threats.

Mrs. Holloway is a schoolteacher during the Cold War who instructs her students, including Jimmy and Eric, on duck-and-cover drills.

David is Michael's younger brother who acts as an intermediary during Michael's stressful baseball game. Unaffected by the superstition of not talking to a pitcher during a perfect game, he ferries messages between Michael and their grandmother.

Carlos is the catcher on Michael's youth baseball team. He attempts to guide Michael through a highly stressful game and encourages him to utilize his curveball.

Coach Clemmons is the coach of Michael's youth baseball team. He notices Michael's anxiety on the mound and reminds him that he does not need to pitch perfectly.

Uncle Dave runs a shop and temporarily houses his nephew, Snider. Recognizing the boy's frustration, he provides Snider with a box of family baseball memorabilia to research, offering him a cut of the profits to motivate him.

Brian is a baseball fan who meets with Snider to purchase an antique bat. He shares memories of attending the last game at Ebbets Field with his late father.