Brierly Ronan was a male individual who held the ranks of colonel and assistant director within the Galactic Empire, specifically under the Imperial Military Department of Advanced Weapons Research. Following his service alongside the Imperial Grand Admiral Thrawn, he consented to become a member of the Chiss Ascendancy, ostensibly as an allied figure, but with the secret intention of turning into an adversary should Thrawn's true objectives diverge from the Empire's.
Colonel Brierly Ronan functioned as the Assistant Director for the Galactic Empire's Project Stardust and displayed unwavering loyalty to Director Orson Callan Krennic. He received orders to collaborate with Grand Admiral Thrawn when grallocs initiated attacks on crucial shipments in the Kurost sector, these shipments being essential for the construction of the Death Star. Throughout their shared assignment, Ronan harbored animosity towards Thrawn, actively attempting to impede the officer's progress in order to secure funding for the Stardust project. However, he gradually developed a degree of respect for Thrawn during their time together. During his service with Thrawn, Ronan became aware of the threat that the Grysk Hegemony posed to the Empire and encountered members of the Chiss Ascendancy. He mistakenly identified Vah'nya, a Force-sensitive Chiss navigator, as a Jedi after observing her reaction to an Imperial soldier's near fall. He also collaborated with Lieutenant Eli Vanto, a former commander in the Imperial Navy and current member of the Chiss Expansionary Defense Fleet, on the mission to Aloxor, with the goal of uncovering the truth behind the attacks on starships by grallocs.
Thrawn had committed to eliminating the grallocs within a week, otherwise, funding for his TIE/d "Defender" Multi-Role Starfighter project would be redirected to Stardust. Ronan witnessed Grand Admiral Thrawn unravel the true conspiracy behind the creature's attacks, revealing it to be the work of Grand Admiral Balanhai Savit. Savit, believing Stardust was doomed to fail, orchestrated the gralloc attacks to enable his operatives to jump away with stolen project components. Thrawn triumphed over Savit in the skirmish against the Third Fleet, but Ronan, still a believer in Stardust and noting that the Chiss officer had not technically destroyed the grallocs, informed Krennic and Grand Moff Wilhuff Tarkin that Thrawn had not succeeded. Subsequently, Thrawn suggested that Darth Vader be assigned to oversee Stardust's security until its initial test, as he was the only Imperial capable of handling the threats to the project. Governor Tarkin concurred with the Grand Admiral's proposal.
Following the discussion with Krennic and Tarkin, Thrawn informed Ronan that he would be unable to conceal his hidden disdain for Emperor Palpatine from Vader as he had from Krennic. Consequently, the Grand Admiral presented him with three options: return to the project and face death at the hands of the Sith Lord for harboring hatred towards the Emperor, resign and attempt to flee to a distant location, or travel with Admiral Ar'alani to join the Chiss Ascendancy. Given his position within the project, Ronan realized that resignation would lead to questioning and, ultimately, death at Vader's hands for abandonment and his negative sentiments towards Palpatine. Despite this, he initially resisted Thrawn's third option. In an attempt to persuade Ronan to join, Thrawn emphasized the threat posed by the Grysks, his experience working with various alien species, the potential value of his insights into the thoughts and actions of others in the impending Grysk war, and the inevitability of his demise if he remained within the Empire. Thrawn also assured him that Ar'alani could return him to the Empire if he proved to be of no use. Ronan, while considering his decision, also recalled Vah'nya and her Force-sensitivity.
Incorrectly assuming that Thrawn and Palpatine had a secret plan to eliminate the supposed Jedi among the Chiss, Ronan speculated that Thrawn might be sending him as a clandestine Imperial asset within the Ascendancy. He then remembered that Thrawn had not been facing him when Vah'nya used the Force and concluded that the Grand Admiral was unaware of his realization regarding her Force-sensitivity. Nevertheless, Ronan believed he could become a secret ally, reasoning that such a role could sometimes be advantageous. Therefore, he agreed to depart with Ar'alani, but only temporarily and only if she could demonstrate that the Grysks posed a threat to the Empire. Thrawn offered Ronan the opportunity to reconsider, but the Assistant Director decided against it, at least for the time being. Furthermore, Ronan reasoned that if Thrawn were genuinely working against the Emperor, he would not be a mere pawn or secret ally. Instead, he believed he could become a formidable adversary. Ar'alani herself considered Ronan's recruitment to be a grave error, although Thrawn viewed the man's reluctance as part of "the framework."
The first appearance of Brierly Ronan was in Thrawn: Treason, a novel authored by Timothy Zahn that was published in July 2019 as the concluding book in the Star Wars: Thrawn trilogy.