Mari Kosan was a human female who held the rank of lieutenant within Saw Gerrera's group of Partisans. Kosan participated in Gerrera's mission aimed at disrupting the Galactic Empire's supply routes on Kashyyyk, and she expressed a desire to remain and support the native Wookiees. When the Empire retaliated against the Partisans, Gerrera determined that staying on Kashyyyk posed too great a risk, while Kosan believed that the Partisans had a responsibility to protect the Wookiee resistance movement.
Prior to the establishment of the Galactic Empire in 19 BBY, Mari Kosan's profession was that of a topographer, creating maps of geological features on remote planets. Although she considered it her ideal occupation, she felt compelled to act against the Empire's transformation of people into "indentured servants," leading her to join the Partisans, a rebel organization under the leadership of Saw Gerrera.
In 14 BBY, the Partisans launched an assault on an Imperial Refinery located on the Wookiee homeworld of Kashyyyk with the objective of disrupting Imperial supply lines. Kosan provided detailed maps of the closest garrisons to the refinery, ensuring that the Partisans were aware that reinforcements would not arrive quickly enough to defend the facility. During an attack on a cargo pad, the Partisans received assistance from Cal Kestis, a Jedi Padawan who was searching for the Wookiee chieftain Tarfful. After Kestis liberated a number of Wookiees at the refinery, Mari waited for them at the cell block's entrance as it was breached, asking the freed Wookiees if they were ready to fight some "imps" and praising Kestis' efforts, indicating that they needed to meet up with Gerrera.
Kestis discovered that Gerrera and his soldiers were trapped and proceeded to use a console to release the remaining imprisoned Wookiees within the refinery. Following the clearing of Imperial forces from the roof, Mari and the others convened with Kestis, Gerrera, and the Wookiees to celebrate the capture of the refinery. After Gerrera's brief celebratory speech, he, Kosan, and the Wookiee Choyyssyk approached Kestis. Gerrera extended an offer to the Jedi to join the Partisans, which he declined, expressing his desire to continue his own mission. Kosan and Choyyssyk then spoke to him about locating Tarfful, promising to stay in contact with his ship, the Stinger Mantis. She conveyed to Kestis that they required the type of leadership that Gerrera provided, acknowledging his unconventional methods but recognizing the severity of the times. Kestis admired her commitment to resisting the Empire, and Kosan agreed that they shared a common cause.
Some time later, after successfully locating Tarfful in the Shadowlands of Kashyyyk, she contacted the Mantis to inform Kestis. By the time Kestis made his way to Kashyyyk, Gerrera had already departed, leaving the remaining Partisans to defend themselves. After battling through the recaptured refinery, Kestis and BD-1 encountered Kosan, Tarfful, and Choyyssyk. Tarfful instructed Kestis to ascend the Origin Tree, and Kosan provided Kestis with a rebreather to aid him in his journey. She informed Kestis that they would continue their fight, as it was the only course of action available to them, and the two groups then separated.
Mari Kosan possessed considerable skill in topography, having created maps of geological formations on various planets before the Empire's rise to power. During her time with the Partisans, she mapped out the garrisons nearest to an Imperial refinery for their benefit. She also had an understanding of the Wookiee language and served as a translator for Choyyssyk's communication with Cal Kestis.
Mari Kosan made her initial appearance in the 2019 video game Star Wars Jedi: Fallen Order. Her voice was provided by Sumalee Montano.