Rick is a 17-year-old from Wisconsin who joins the military to avoid taking over his family's modest hardware store. Frustrated by teachers and family members who view him as a "generalist" lacking a specific calling, he hungers to prove his bravery and capability as a specialist. He uses a family connection to become a dog handler and is paired with an initially hostile German Shepherd named Cracker, whom he determines to train into the best scout dog in Vietnam.

Cracker is a large, intelligent German Shepherd who originally trained as a show dog before breaking her leg. She belongs to a 10-year-old boy named Willie until his family moves to a dog-free apartment building. Reassigned to the US Army's dog handling division, she exhibits fierce aggression toward strangers and resists initial training efforts until she slowly begins to bond with her new handler, Rick.

Cody is a fellow soldier in the dog handling division who quickly excels in the training program. He possesses significant experience with dogs and easily commands his partner, Bruno, earning Rick's early envy. He develops a close bond with Rick and Twenty-Twenty as they train and deploy together.

Twenty-Twenty, often called Twenty, is a soldier in the dog handling unit paired with a dog named Tristie. He forms a tight trio with Rick and Cody during their time at Fort Benning and their subsequent deployment. He comes from a military background and occasionally references a decorated uncle to excuse minor infractions.

Willie is a 10-year-old boy who deeply loves his German Shepherd, Cracker. When his parents relocate the family to an apartment building that bans dogs, he makes the painful decision to surrender her to the US Army's war dog program. He maintains hope that she will succeed in the military and writes frequent letters to her new handler, detailing her intelligence and vocabulary.

Bruno is an exceptionally well-behaved military dog assigned to Cody. Rick privately refers to him as "the Mr. Universe of dogs" due to his impressive physical discipline, muscular build, and immediate synchronicity with his handler.

Tristie is the military dog assigned to Twenty-Twenty. Together with her handler, she forms the second-best team at the Fort Benning camp, providing a benchmark of excellence that Rick and Cracker strive to surpass during their preparation for deployment.

Sergeant "U-Haul" is the acerbic, demanding drill instructor in charge of Rick's unit. He is exceptionally harsh toward Rick, frequently singling him out for public criticism and discipline. Underneath his punishing exterior, he closely monitors the progress of his canine and human recruits to prepare them for the realities of war.

Willie's father makes the practical decision to move the family to a new apartment that does not permit dogs, forcing the surrender of Cracker. He physically leads Cracker out of the house to hand her over to the Army recruiter, focusing on the practical realities of their housing situation.

Willie's mother opposes the war but supports the decision to give Cracker to the Army due to their housing situation. She attempts to comfort the dog on the morning of her departure, expressing sympathy for the animal while bound by practical constraints.

Rick's father owns a modest hardware store in Wisconsin. He views his son as a reliable boy whom he expects to inherit the family business, a perception that inadvertently frustrates Rick and drives his desire to join the Army to prove he is capable of more.

Rick's sister studies mathematics at the university. Her clear academic calling and intelligence serve as a point of comparison for Rick, who feels inadequate next to her clear specialization and success.

Camel is a member of the elite Special Forces unit operating in Vietnam. He helps train Rick and Cracker on new equipment and coordinates specialized, highly dangerous missions involving enemy territory that require the tracking skills of a dog.

Madman is a Special Forces soldier who works alongside Camel. He participates in high-stakes, specialized operations and works directly with Rick and Cracker when they are assigned to assist his elite unit.

Rafael acts as Rick's "slack-man" during missions in the field. His primary duty is to watch Rick's back and provide security so that Rick can focus his attention entirely on interpreting Cracker's subtle signals for traps or enemy presence.