Brian is a fourteen-year-old baseball savant who lives in Bloomfield Hills, Michigan. He plays outfield for his Little League team, the Sting, and possesses an encyclopedic knowledge of baseball statistics. Working as a batboy for his favorite team, he faces demanding hours and difficult physical tasks but approaches them with extreme dedication. He deeply misses his absent father and seeks a positive male figure in the wake of his parents' divorce.

Hank is a 35-year-old former Most Valuable Player whose career has been derailed by poor choices, including fighting and two steroid suspensions. Returning to the Detroit Tigers to serve as a backup designated hitter, he approaches his work with a surly, bitter attitude. He is trying to reach the milestone of 500 career home runs to prove his continuing worth to the sport and the public.

Cole is a former Major League relief pitcher who spent 14 seasons moving between ten different teams. Following his retirement, his inability to adjust to a life without baseball led to his divorce from Liz. He currently works as a baseball coach in Japan and struggles to communicate with his son on any topic other than the sport.

Liz is a senior producer and writer for an all-news radio station in Detroit. After her marriage to Cole ended because of his obsession with baseball, she views the sport as a cumbersome foreign language. Despite her reservations, she is a deeply supportive mother who adjusts her work schedule to accommodate Brian's demanding batboy duties.

Willie is the friendly, humorous shortstop for the Detroit Tigers. He rebels against the team's strict health food diet by paying Brian to sneak him fast food from McDonald's. Unlike some of his teammates, Willie treats the stadium staff well and offers thoughtful perspectives on the sport's controversies.

Finn is a young batboy for the Detroit Tigers who works closely alongside Brian. He takes a pragmatic approach to the job, believing that batboys should remain as unobtrusive as possible. He serves as a sounding board for Brian at the stadium and holds a highly critical view of Hank Bishop's toxic attitude.

Kenny is Brian's best friend outside of the Major League stadium environment. He is a talented pitcher for their Little League team, the Sting. Kenny provides an outside perspective on Brian's deep immersion in the baseball world, occasionally warning his friend about the dangers of letting the sport consume him entirely.

Jim is the clubhouse and equipment manager for the Detroit Tigers. Impressed by Brian's persistent letters and high academic grades, he advocated for the fourteen-year-old to get the batboy job despite his young age. He oversees the boys' duties and observes the shifting moods of the professional players.

Davey is the manager for the Detroit Tigers. He maintains a good-natured presence in the dugout and is observant enough to joke with Brian about the young batboy's sharp intuition regarding on-field baseball strategies.

Curtis is a strikeout-prone player for the Detroit Tigers. Despite being considered an inept bunter, he successfully executes a play that allows a teammate to score a winning run early in the season.

Mary is an outfielder for the Detroit Tigers who interacts warmly with visitors in the clubhouse. Mary notices the hard work of the stadium staff and openly praises Brian's work ethic to his visiting father.