Juan "Johnnie" Rico is a young man from a wealthy family who goes against his father's wishes to join the Federal Service. Initially motivated by the desire to earn his citizenship and voting rights, he finds himself assigned to the elite and dangerous Mobile Infantry. As he undergoes rigorous basic training, he learns the value of discipline, duty, and brotherhood while surviving harsh physical trials.

Mr. Dubois is a retired Mobile Infantry officer who serves as a cranky and demanding high school teacher. Missing part of one arm, he challenges his students to understand the price of citizenship and the moral foundations of society. His philosophical arguments act as a guiding voice for his students as they enter the military.

Sergeant Zim is a tough, highly skilled, and widely feared basic training instructor at Camp Currie. He pushes recruits to their absolute physical and mental limits to prepare them for the lethal realities of the Mobile Infantry. Behind his harsh disciplinary exterior, he possesses a deep sense of responsibility for the men under his command.

The Bugs are a hostile, insect-like alien species at war with humanity. They operate as a communal hive entity with no regard for individual lives, relying on overwhelming numbers and strict caste divisions between workers and warriors to overwhelm their enemies. They control the planet Klendathu and constantly expand their interstellar territory.

The Brain Caste forms the elite leadership tier of the Bug civilization. They reside deep within fortified hive networks, controlling the worker and warrior masses while devising strategies against the Terran Federation. Capturing them is a primary, highly dangerous objective for Earth's intelligence forces.

Captain Blackstone is the commander of the Mobile Infantry company nicknamed "Blackie's Blackguards." He oversees field training for officer candidates, pushing them to delegate properly and balance their heavy workloads. He demands absolute competence while ensuring his junior officers do not burn themselves out from stress.

Sergeant "Jelly" Jelal is a small but incredibly tough platoon sergeant in the Mobile Infantry. He handles the practical, day-to-day operations and combat deployments of his unit. He keeps a careful watch over his soldiers and guides their career paths from behind the scenes with a seasoned eye.

Emilio Rico is the wealthy head of a major corporation and the patriarch of a family that has avoided politics and military service for a century. He expects his son to inherit his business empire and views the Federal Service as a peacetime anachronism. His rigid opposition leads to a bitter, prolonged rift in the family.

Carmencita Ibañez is a petite, brilliant, and highly athletic high school graduate who signs up for the Federal Service. Possessing perfect reflexes and a talent for mathematics, she aims to become a spaceship pilot in the Terran military, representing the high demand for capable female pilots in the fleet.

The Skinnies are tall, slender, humanoid aliens who are militarily weaker than both Earth and the Bugs. They are initially allied with the insectoid enemy, prompting the Federation to launch aggressive combat drops to pressure their government into switching sides in the interstellar conflict.

Ted Hendrick is an earnest but undisciplined recruit in the Mobile Infantry. His inability to follow strict orders during a field exercise leads to a severe physical altercation with an instructor, demonstrating the military's harsh code of justice and strict disciplinary boundaries.

N. L. Dillinger is a recruit who deserts basic training and commits a horrific crime in civilian society. The military takes custody of him to administer formal justice, illustrating the uncompromising nature of the Federal Service's disciplinary system and its commitment to taking responsibility for its own.

Carl is a high school student with a strong aptitude for electronics and science. He enlists in the Federal Service alongside his friends, specifically requesting and receiving an assignment in research and development at an off-world station.

Lieutenant Rasczak is the commanding officer of an elite Mobile Infantry company. He maintains a friendly but remote demeanor in daily life, but during battle, he demonstrates an extraordinary ability to track and worry over the disposition of every single man under his command.

Captain Frankel is a battalion commander who oversees basic training. He customizes instruction to individual students and deals directly with disciplinary issues, balancing the need for a strict military code with an understanding of his sergeants' heavy burdens.

Breckinridge is a huge young recruit with a Southern accent who accepts a physical challenge from his instructor on the first day of basic training. He quickly learns the harsh gap between raw physical size and true martial arts expertise.

Shujumi is a short, highly skilled recruit and the son of a colonel. His extensive background in martial arts earns him the respect of his trainers and leads to him being utilized as an impromptu instructor for his struggling peers.

Ace is a senior corporal in the Mobile Infantry who initially struggles with taking orders from newly promoted superiors. He operates by an old-school, rough-and-tumble code of respect among the enlisted men, requiring physical confrontation to settle disputes.

Major Reid is a blind officer who teaches History and Moral Philosophy at Officer Candidates School. He challenges his students with complex ethical scenarios, forcing them to articulate the deep responsibilities tied to governance, citizenship, and military force.

Commandant Nielssen is a disabled officer who oversees cadets at Officer Candidates School. He provides critical advice on leadership, warning new officers about the heavy burdens of command and the necessity of trusting experienced non-commissioned officers in the field.

Birdie is a brilliant young cadet and math genius with extensive combat experience. His peers view him as a natural leader with the intellect and common sense necessary to eventually reach the rank of general.

Jenkins is a trainee who struggles with the intense physical demands and strict discipline of Camp Currie. His sheer exhaustion highlights the brutal nature of basic training, as he relies on the kindness of his fellow recruits to keep his strength up.

Captain Deladrier is the highly skilled naval commander of the spaceship Rodger Young. She executes complex drop and retrieval maneuvers with precision, earning the deep respect and gratitude of the infantrymen she transports into battle zones.