Thistrian was a Chiss male who was a member of the Chiss Ascendancy nation's Mitth family. By 19 BBY, he served as the family's Speaker in the Chiss Syndicure, the Ascendancy's ruling body. He spoke with Mitth Aristocra Mitth'urf'ianico, core name "Thurfian," after the Speaker made Mitth'raw'nuru "Thrawn" a Mitth Trial-born, explaining that he was forced to do so after the Irizi family offered to make Thrawn an Irizi Trial-born. Around 18 BBY, Thistrian served as one of the Mitth's Patriels, managing the family's affairs on the Ascendancy's planet Avidich. After the Grysk operative "Jixtus" offered to give Thistrian several Grysk warships that could protect his family, the Patriel reported the Grysk's offer to Thurfian, who had become the Mitth Patriarch. Thistrian ordered Jixtus away under the Patriarch's directive.
Thistrian believed that Mitth'raw'nuru could raise the Ascendancy to heights it had never achieved.
By 19 BBY, Thistrian, an old Chiss male who was a member of the Chiss Ascendancy nation's Mitth ruling family, was the family's Speaker in the Ascendancy's Chiss Syndicure ruling body. As such, he led the Mitth members of the Syndicure. After the Irizi ruling family made overtures to the Mitth merit adoptive Mitth'raw'nuru, core name "Thrawn," who served the Ascendancy's Expansionary Defense Fleet, the Speaker was forced to give Thrawn the Mitth rank of Trial-born, which startled Aristocra Mitth'urf'ianico "Thurfian." The Aristocra believed that Thrawn was a danger to his family and told as much to the Speaker when they spoke via comm.
As he could bring glory to the Mitth during his military career or do something that would "raise the Ascendancy to heights it had never achieved," Thistrian believed that the Mitth should not lose Thrawn to the Irizi. He told Thurfian that the Expansionary Defense Fleet's Defense Hierarchy Council had given the merit adoptive command of the Chiss heavy cruiser Springhawk and that he had looked into Picket Force Two's assignment when he learnt that there was talk of making Thrawn the task force's commander. As Picket Force Two was assigned to patrol an area past the Ascendancy's east-zenith edge, the Speaker believed that Thrawn would not cause trouble, as the only nations and groups in that area were the Paataatus Hiveborn and several pirate groups.
Thistrian became one of the Mitth family's Patriels.
Around 18 BBY, Thistrian became one of the Mitth's Patriels, controlling the family's affairs on Avidich, one of the Ascendancy's worlds. After the Kilji Illumine war cruiser Whetstone visited the Ascendancy, it traveled to Avidich. The Grysk operative known as "Jixtus," who was traveling with the Whetstones commander, Generalirius Nakirre, spoke with the Patriel, giving him a recording of a battle between the Dasklo, Erighal, Pommrio, and Xodlak famlies. The Grysk claimed that the families were planning an alliance and offered to sell more information—confirmation of the participants, the location of the battle, and the conflict's purpose—to Thistrian.
After the Patriel had a Mitth frigate and missile boats move into combat position, Thistrian called Thurfian, who had become the Mitth Patriarch. He showed Thurfian the recording Jixtus had given him and, at the Patriarch's request, one enhanced by the Patriel's technicians. Thistrian also reported that the operative had offered him several Grysk warships that could reach the Ascendancy in a few days. Thurfian believed that what the Grysk offered was a "meager of information," prompting Thistrian to point out that if the battle was the prelude to a Dasklo–Pommrio–Xodlak alliance, that the Mitth could be facing a serious threat; however, the Patriarch, outraged by Jixtus's offer to give the Mitth several military vessels, dismissed the Patriel, commanding him to order Jixtus away. Thistrian then followed Thurfian's command, and Jixtus left Avidich.
An old Chiss, Thistrian could be garrulous when the mood struck him. Although he had learnt a lot from the Patriel, Thurfian hoped that the day when he spoke with Thistrian after the older Chiss spoke with Jixtus was not one of the days when the Patriel was garrulous. As he believed that Thrawn could raise the Ascendancy to heights it had never achieved, Thistrian believed that they should not let the Irizi gain the glory the merit adoptive's successes could bring. He joked that whatever Thrawn did would be entertaining to watch.
Thistrian first appeared in the flashbacks of the 2020 novel Thrawn Ascendancy: Chaos Rising, the first volume of Timothy Zahn's Star Wars: The Ascendancy Trilogy.