Sarin was a female Twi'lek of the rare Lethan variety. During the Galactic Civil War, she operated as an undercover agent for the Alliance to Restore the Republic.
Possessing the rare Lethan gene made Sarin a particularly effective Rebel operative. She was able to maintain her deep cover on the planet Feddasyr for a duration of 5 years without detection.
During a mission aimed at acquiring a DSS-02 shield generator, Sarin was dispatched to the planet Feddasyr. Her assignment was to gather intelligence concerning the independent industrialist known as Jock Tardee. However, her identity was compromised, forcing her to seek refuge before her scheduled rendezvous with her Rebel contact. Upon arriving at the cantina, the designated meeting point, Sarin discovered that an imperial Cerean agent was present. Fortunately, before the Cerean could act, he succumbed to poison administered to his drink by Sarin's contact, Leia Organa. Sarin proceeded to Leia's penthouse, providing her with all the data collected on their target, and then received instructions from the princess to provide overwatch.
Later that evening, Leia, using the alias Nahob Skuts, infiltrated Jock Tardee's party. While attempting to locate the imperial codes that Jock Tardee had acquired, an imperial Trandoshan agent, who was also after the codes, recognized Leia Organa. However, before the Trandoshan could expose the princess, Sarin intervened, eliminating him with a sniper rifle that Leia had left on the penthouse balcony. After securing a stormtrooper defector, who had the imperial codes encoded within his DNA, Leia successfully escaped the party and met up with Sarin. Leia entrusted the stormtrooper to Sarin's care before being apprehended by an imperial patrol. Sarin evaded capture and connected with her sister, also a member of the Alliance. Sarin conveyed to her sister that the stormtrooper was "of critical importance to the cause" and needed to be delivered to the next rebel contact on a remote planet.