Lady Thalassa Tarkin, a Human female hailing from the powerful Motti family of Phelarion, was the spouse of Grand Moff Wilhuff Tarkin. Wilhuff Tarkin's marriage to Thalassa was a strategic move for political gain, devoid of genuine affection; he engaged in an extramarital relationship with Natasi Daala while still married. Nevertheless, Thalassa Tarkin consistently supported her husband and the Empire, even following his death on the Death Star. Subsequently, she oversaw a megonite moss operation on Phelarion, aiming to provide ammunition to the Empire to further suppress the Rebellion. Shortly before the Thirteenth Imperial Diplomatic Conclave, which Lady Tarkin hosted, Princess Leia Organa infiltrated Tarkin's slave operation, disrupting the planned megonite transaction with the Empire. Darth Vader, present on Phelarion for the conclave, held Lady Tarkin accountable for the disruption.
Lady Thalassa Tarkin, a wealthy member of the Motti family from Phelarion, entered into marriage with Imperial Governor Wilhuff Tarkin. Wilhuff Tarkin's motivation for marrying her stemmed from her family's influential connections, notably her cousin, Commander Conan Motti. In reality, Wilhuff Tarkin harbored no love for her; soon after their wedding, he initiated an affair with Natasi Daala, a young cadet stationed on Carida.
Despite the Governor's lack of affection for his wife, they had a son named Garoche, who later became an admiral within the Imperial Navy. In 19 BBY, Garoche secretly betrayed the Empire by defecting to the Atoan system and assisting their insurgency against the Galactic Empire. Official reports indicated he went missing and was subsequently killed by Atoan natives. However, the true perpetrator was Darth Vader, Emperor Palpatine's right hand man. Vader, along with his master, believed that Garoche's death would transform Governor Tarkin into a formidable, hate-fueled ally who would show no mercy to the Empire's adversaries. Their scheme proved successful.
In the post-Clone Wars era, the Tarkin family on Governor Tarkin's homeworld of Eriadu had established a reputation as a force to be reckoned with. When members of the Nebula Front abducted Lady Tarkin's niece, Shella Motti, the Tarkins enlisted agents to locate the missing girl; the Mottis, however, hired their own agents to prevent her return to her family. In 5 BBY, following the death of Lady Tarkin's brother-in-law, Gideon Tarkin, during the Erhynradd Mutiny, his daughter Rivoche was invited to reside at the Tarkin estate. However, the rebellious teenage Rivoche was eventually sent to an exclusive preparatory academy on Clær due to her frequent outbursts.
Around 1 BBY, Thalassa Tarkin and her husband resided in an apartment on Coruscant. It was in this apartment that Thalassa learned of the Death Star, a moon-sized battle station constructed by the Empire. Her husband informed her of his new permanent assignment on the Death Star and showed her a hologram of the superweapon.
Although Grand Moff Tarkin perished aboard the Death Star when the Rebel Alliance destroyed it, Imperial news outlets reported his death as a result of a shuttle accident at the Tallaan shipyards. Lady Tarkin, aware of the truth, became consumed with avenging his death by crushing the Rebellion. At Port Tarkin on Phelarion, she commissioned the construction of a solid black monument in his memory. The plaque affixed to it read:
Lady Tarkin inherited a megonite moss mining industry on Phelarion, established by Lithio Motti centuries earlier. Tarkin managed the business as an Imperial labor colony, utilizing slaves to mine the explosive, heat-sensitive moss for the production of ammunition for the Empire, to which she remained fervently dedicated, particularly in their efforts to eradicate the Rebellion. Shortly before the Thirteenth Imperial Diplomatic Conclave, which Lady Tarkin was hosting, Princess Leia Organa found herself stranded on Phelarion. Mistaking her for a slave girl, Tarkin selected Organa, whom she referred to as "Lerna," to serve as a servant during the conclave. After instructing an Imperial freighter to postpone its megonite pickup due to an explosion at the mine, Tarkin noticed a significant discrepancy in the megonite inventory and began to suspect theft. Subsequently, Darth Vader contacted Tarkin regarding the delayed landing, accusing her of selling the megonite to the Rebel Alliance. Enraged, Tarkin resolved to uncover the individual responsible for stealing her megonite.
Upon returning to her private chambers after hearing a loud noise, Tarkin discovered her servant Nardo dead from an explosion within her megonite display case. Nevertheless, she proceeded to her banquet hall to host the Imperial Diplomatic Conclave, attended by guests such as Shea Hublin, Chancellor Qua of the Zenox Star Cluster, the Most High Proctor of Thebeon 8, and Darth Vader himself. An awkward moment occurred when Tarkin offered a drink to Vader, who reminded her that he neither ate nor drank. Vader grew impatient with Tarkin, reiterating his accusation of selling to the Rebels. His stormtroopers then discovered the crashed Rebel glider that Organa had used to land. When Tarkin spotted her servant girl Lerna and two other slaves fleeing the scene, Vader recognized the girl as Princess Leia Organa. While Tarkin doubted the Rebels' chances of escaping the planet, Vader vowed to hold Tarkin personally responsible if Organa managed to escape. Ultimately, the Princess successfully departed from the planet.
Thalassa Tarkin was a stern and dignified woman who held her husband in high esteem. Despite Wilhuff Tarkin's affections lying elsewhere, Lady Tarkin regarded him as a martyr and hero, remaining loyal to his cause. She took pride in her dedication to the Empire and was deeply offended by Darth Vader's accusation of selling war matériel to the Rebellion. Tarkin was also paranoid, believing that Rebels were lurking everywhere, seeking to harm her. Despite her paranoia, she never suspected that her handmaiden Lehra was not only a Rebel agent but also Princess Leia.
Lady Tarkin made her initial appearance in Princess Leia, Imperial Servant, a 1979 newspaper strip by Russ Manning. Three decades later, in 2009, Nathan O'Keefe bestowed upon her the first name "Thalassa" in his short story, "Tinker, Tailor, Soldier, Praji." Thalassa, meaning "Sea" in Greek, was the name of a primordial sea goddess in Greek mythology, as well as a moon of Neptune. O'Keefe selected the name because it sounded "cool and Star-Warsy," rather than for any thematic connection to its existing meanings.