Specially trained by the First Order, Agent Tierny hopes to restore peace to the galaxy through absolute order. Tierny is a master of manipulation, tactically luring her targets into a false sense of security rather than using more aggressive forms of interrogation. Her arrival at the Colossus platform, as part of a First Order security takeover, ushers in sheer chaos when the platform is plunged deep into the waters of Castilon, leaving only Doza Tower above sea level.
Tierny later recruits Colossus residents Tam Ryvora and Jace Rucklin to the First Order via manipulation, telling them that the Republic is evil and that the First Order only wants to bring peace to the galaxy via absolute control. As the First Order is being chased off the Colossus, Tam escapes with Tierny and Commander Pyre.
After the death of Lieutenant Galek, Tam is promoted to the leader of her squadron by Tierny. However, Tam decides to defect from the First Order after witnessing them massacre the inhabitants of the planet Aeos as a show of force because the Colossus was formerly there. No longer content with murdering innocent people, Tam leaves with Rucklin under the guise of "training", although she is actually planning to meet Kaz and Yeager.
Tierny and Pyre report to General Hux, only to be met by Supreme Leader Kylo Ren. He tells them both that he is displeased with their failure and tells them that in the future there will be consequences. He then uses the Force to make them both draw their blasters and point them at each other, but lets them go at the last second.
After failing to stop Tam's escape, Tierny is contacted by Kylo Ren. She asks him to send reinforcements to capture the Colossus, but he refuses, saying that doing so will only add to her failure. Tierny tells him she understands, to which Ren responds "Do you?" before Force choking her to death.
- The original outline of "Descent" had Commander Pyre as the one with the task to arrest and question Tam Ryvora instead of Tierny. In the episode's first draft, rather than an agent, Tierny was an admiral, but by the second draft, Tierny was redefined into an interrogation agent. Much of Tierny's look was inspired by real-world police gear, with her helmet being a mash-up between those worn by Darth Vader and Agent Kallus.