Sergeant Dunn serves as a sergeant in the Mobile Infantry unit Willie's Wildcats. He greets new recruits Juan Rico, Smith Alphard, Pat Leivy, and T. Azuma upon their arrival at an orbital base, immediately instructing them to inspect and maintain their individual drop capsules, known as "coffins." Aboard the spacecraft *Rodger Young*, he emphasizes strict adherence to protocol and naval etiquette.

During the Battle of Glendathu, Dunn commands the Wildcats against enemy forces. Spotting an enemy stronghold, he orders his troopers to lay down covering fire while he advances alone to designate the target for an incoming bombardment squad. Disregarding his explicit orders, most of his soldiers follow him into the hostile structure, leading to Dunn's capture by enemy combatants. Juan Rico rescues him during the Wildcats' assault. Following his rescue, Dunn coordinates with Rico, Kitten Smith, Pat Leivy, and Sergei Cherenkov to launch a full-scale counterattack.

Dunn leads with a pragmatic, mission-focused style, issuing practical, task-oriented commands. He prioritizes efficiency and expects disciplined obedience, demonstrating attention to concrete details during troop training and mission preparation. Before combat deployments, he reminds soldiers of their duty to conserve Federation resources by surviving. His survival status following the Battle of Glendathu is unconfirmed.