Mirienna was a human female resident of the planet Zeffo during the Imperial Era. When the Galactic Empire occupied Zeffo for their Project Auger, displacing the citizens of a Zeffo village, Mirienna's husband Ostar led several stormtroopers away so that she could escape with her daughter Sienna. Her husband was killed in his attempt, and Mirienna joined Saw Gerrera's Partisans, sending her daughter to safety on Chandrila, though she later decides to reunite with Sienna.
Mirienna was a human who lived with her husband, Ostar, and their infant daughter, Sienna, in a village on the planet Zeffo. By 14 BBY, the Galactic Empire had occupied Zeffo in order to carry out Project Auger and displaced the occupants of the village. Ostar distracted a group of stormtroopers so that Mirienna could escape with Sienna, and the family became separated, leaving a bag behind in the village. Mirienna and Sienna ended up hiding in a cave tunnel that linked the village to a Derelict Hangar through a mountain. In the cave, Mirienna used a piece of fabric as a blanket to keep a crying Sienna warm, but left the blanket in the cave before escaping Zeffo without Ostar, not knowing if her husband was alive or dead.
Mirienna then sent Sienna to be with her father on the planet Chandrila. The mother's battle against the Empire then took her across the galaxy and she signed up with the rebel Saw Gerrera's Partisans. She fought alongside Gerrera on the planet Kashyyyk, where the Empire had enslaved the native Wookiees and assisted in successfully freeing Wookiee prisoners that were being held in an Imperial Refinery with aid from the fugitive Jedi Cal Kestis.
Mirienna received an encrypted transmission from her father letting her know that Sienna had safely arrived on Chandrila. Kestis approached her on the captured Cargo Pad 119-Grek and asked if she was alright, and Mirienna explained the message she had received and called it the best news she had had since arriving on Kashyyyk. Kestis then revealed that he had seen visions of Ostar on Zeffo and found his corpse, finally confirming her husband's fate. Kestis apologized for delivering such bad news, but Mirienna told him she was glad to know the truth.
When Kestis returned to Kashyyyk having received confirmation that Tarrful would meet with him, Mirienna was pleased to see he was safe and informed him that a map to the rendezvous point had been drawn up for him by Mari Kosan and Choyyssyk. She confirmed that Gerrera has retreated offworld and that their casualties had reached an unsustainable level. She knew the cause was always bad under the Empire, but told Kestis their was still a chance for his quest to succeed. She thanked him for his efforts in the fight, and informed him that she would be leaving Mari Kosan's cell to raise Sienna while finding other ways to fight the Empire.
Mirienna was a brave fighter but missed her daughter while fighting alongside Gerrerra. After becoming separated from Ostar, part of her wanted to believe he had escaped the Imperials, but when Kestis confirmed his death, she was glad to know the truth. She believed that the Empire could not be allowed to destroy any more worlds. After learning her husband's death, she decided to leave the battlefield to raise her daughter while continuing to find other ways to fight. Mirienna had dark skin, black hair, and brown eyes.
While on Kashyyyk, Mirienna wore a yellow and tan jacket with gray pants, black boots, and a brown backpack. She used a DC-15A blaster carbine.
Mirienna appeared in the 2019 Respawn Entertainment video game Star Wars Jedi: Fallen Order.