Nantz held the rank of admiral within the Alliance to Restore the Republic's fleet during the Galactic Civil War, a conflict waged against the Galactic Empire. Sometime after the Battle of Hoth, they were present at a secret assembly of the Alliance's high command within a Rebel safe house situated on the planet Zastiga. During this meeting, Admiral Gial Ackbar disclosed the Empire's construction of a second Death Star orbiting the moon of Endor. Nantz engaged in discussions regarding the necessary personnel for an assault on the Death Star, emphasizing the critical need for a diversion to allow Alliance forces to mobilize undetected by the Empire. Princess Leia Organa proposed leading a decoy mission to the Corva sector to create this diversion. This distraction proved successful, enabling the Alliance fleet to assemble and ultimately destroy the Death Star during the Battle of Endor. Nantz possessed a stern visage, described as grim and hatchet-like.
Nantz's initial mention occurred within the Star Wars Legends novel titled The Black Fleet Crisis, authored by Michael P. Kube-McDowell and published in 1996. The Legends reference book The Essential Guide to Warfare, penned by Jason Fry, identified Nantz's given name as Firmus. Furthermore, the book's endnotes revealed that a previously unnamed character featured in the comic "Lando's Commandos: On Eagles' Wings" was, in fact, the aforementioned Admiral. Nantz's presence within canon was affirmed by the novel Moving Target: A Princess Leia Adventure, co-written by Fry and Cecil Castellucci and released in 2015.