5-L was a MSE-6 series repair droid in service to the First Order during their war against the Resistance. DT-533, a TIE pilot, wanted to desert the First Order and thusly reprogrammed 5-L to aid her in this endeavor.
The year 34 ABY found 5-L serving the First Order on the Resurgent-class Star Destroyer called the Thunderer. Not long after the suppression of the Aeos system, 5-L went to the barracks on the Thunderer where First Order pilot DT-533, whose real name was Tamara Ryvor, resided, in order to perform system updates. DT-533 asked the droid to postpone the upgrades, but the droid continued with its task. As the pilot urged the droid to continue the upgrade and moved closer, 5-L attempted to escape. DT-533 trapped 5-L by closing the door and then picked up the mouse droid. After taking 5-L to a bunk, the pilot opened the droid's panels and reprogrammed it to help her. She reassured 5-L after the reprogramming, telling it that it would assist her in escaping.
Later, as Commander Pyre and Agent Tierny were leaving Tierny's office, 5-L was placed in the doorway, preventing the door from closing. Ryvora entered the office and moved the droid from the door. She thanked 5-L as she searched Tierny's desk. After getting her comlink from the locked drawer, she heard Tierny returning. She told 5-L to hide as she stood by the door. 5-L failed to hide before Tierny came into her office. Tienry was looking at a datapad, allowing the pilot to sneak out unnoticed. Tierny noticed 5-L in the middle of her office and asked if the system upgrade was done. 5-L chirped in response before leaving the office and meeting Ryvora in the hall.
Ryvora took 5-L back to her quarters and opened the droid's interior again. She connected her comlink to one of 5-L's inner wires and sent a message to Resistance member Kazuda Xiono's comlink, requesting a meeting on Castilon in three cycles. The message was sent in a mechanic's code that she and Neeku Vozo had created.
5-L was a MSE series droid with black plating on its exterior.
5-L's debut was in the first part of "The Escape," a two-part episode that served as the finale for the second season of the animated TV show Star Wars Resistance, which was broadcast on January 26, 2020. Aaron Pham, an artist at Lucasfilm Animation, created concept art for 5-L on August 21, 2018. The droid's designation is a nod to Fievel Mousekewitz, the main character from the 1986 animated film An American Tail, which was produced by Amblin Entertainment.