Dennion served as a Human intelligence operative for the Rebel Alliance during the Galactic Civil War that saw them pitted against the forces of the Galactic Empire. Initially, Dennion was forced into service by the Empire to suppress insurgencies on their homeworld, under the belief that conformity was essential for survival. The deaths of comrades who resisted Imperial rule prompted a change of heart, leading Dennion to defect and join the Rebellion. In one operation, Dennion spearheaded a mission to Canto Bight with the objective of extracting fellow rebel Asara Deyn from the mansion belonging to Sliro Barsha of the Zerek Besh syndicate. To infiltrate the building, they enlisted the services of the rogue Kay Vess. However, after Deyn's successful retrieval, Vess, unconvinced of the rebels' ideals, demanded immediate payment, leading them to stun her and leave her behind.
During the Imperial Era, Dennion was drafted into the ranks of the Galactic Empire, tasked with combating insurgent forces on their homeworld. Dennion's initial conviction was that compliance was necessary for survival. However, the demise of friends who opposed the Empire altered this perspective. Subsequently, Dennion abandoned their allegiance and joined the Alliance to Restore the Republic.
In the year 3 ABY, a few months following the Battle of Hoth, Dennion was coordinating the extraction of Twi'lek rebel Asara Deyn, who was being held within the vault of Westhill Palace. This mansion was the property of Zerek Besh crime boss and Imperial Security Bureau director Sliro Barsha. With the assistance of fellow rebels and the recruitment of Human scoundrel Kay Vess, they launched an assault on Westhill Palace. While the main rebel force attacked the outer defenses, creating a diversion, Kay Vess, possessing the access codes to the vault, infiltrated the palace and fought her way to the vault's location.
Kay successfully gained access to the vault, believing the mission was to pilfer credits, unaware of Dennion's rebel affiliations. Kay was taken by surprise when the vault she opened contained Asara Deyn. Dennion and the other rebels soon arrived to meet Dennion, but Kay held them at gunpoint and demanded payment. Even as Asara attempted to persuade Kay to embrace the Alliance's cause, Dennion, deeming the situation too urgent for persuasion, opted to [stun](/article/stun] Kay before the rebels made their escape from the palace.
Dennion and Kay Vess crossed paths once more when Kay inadvertently stumbled upon their hidden Rebel base situated on the Outer Rim jungle planet Akiva. Kay was there searching for components to repair her damaged BX-series commando droid, ND-5. Dennion offered an apology to Kay, admitting their initial assessment of her had been incorrect. Kay, however, dismissed the apology, stating that the situation might have unfolded differently had Dennion been forthright about the true objective of the heist.
Later, when Kay Vess, her crew, and Asara were escaping the Imperial-class Star Destroyer Revelator, Asara attempted to contact Dennion and the base, warning them that the Empire knew their location and urging them to evacuate. Witnessing this, Kay decided against fleeing the battle aboard the Trailblazer and instead turned back to confront the Revelator, while simultaneously sending out a distress beacon to request assistance from a crime syndicate with whom she had a positive relationship. Dennion communicated with Vess and others throughout the battle. These efforts proved fruitful, and with the combined forces of syndicate starfighters summoned by Kay and Rebel T-65B X-wing starfighters deployed by Dennion, the Revelator was destroyed, securing the safety of the Akiva base.