Nomen Karr, also referred to as the Sith Lord Retrost during his clandestine operations, was a male Human Jedi Master. He stood as one of the most experienced veterans within the ranks of the Jedi Order throughout the duration of the Cold War that transpired between the Republic and the Sith Empire. In addition, Karr served as the personal adversary to the Sith Lord Darth Baras. A valiant Jedi, Karr was known to brush against the dark side of the Force while operating within the Sith Empire's territory, yet he consistently remained loyal to the tenets of the Jedi Code. Driven by his determination to expose Imperial spies within the Galactic Republic, Karr took on Jaesa Willsaam as his Padawan, providing her with training. His actions drew the attention of Darth Baras, who dispatched his latest apprentice, the future Emperor's Wrath, with the objective to destroy him.
Being a Force-sensitive Human, Nomen Karr received instruction in the ways of the Force from the Jedi Order, eventually achieving the rank of Jedi Master and constructing his own lightsaber with a green blade. While appreciated for his extensive experience, Nomen Karr was also recognized for questioning the stringent precepts of the Order as he waged his personal war against evil throughout the galaxy. With the Sith Empire's resurgence in Republic space, Karr operated covertly behind Imperial lines under the guise of "Retrost," aiming to gather intelligence concerning the Empire and its Sith inhabitants. The resolute Karr even ventured to touch the dark side during his time within Imperial territory, yet he emerged as a courageous champion of the light. He held a firm belief that the Republic had been infiltrated by the Sith Empire and sought to prove the Jedi High Council wrong for placing trust in a Sith's word. Karr's endeavors ultimately led to the capture of Darth Ekkage, Baras's sister. Before their duel on Nal Hutta resulted in a stalemate, Baras managed to unmask Karr, causing the latter to flee. Karr's betrayal created a deep-seated animosity between the two.
Driven by the desire to eliminate the Imperial infiltrators he believed were concealed within the Republic and Jedi ranks, Karr dedicated himself to locating and bringing these spies to justice, despite Baras's persistent attempts to thwart his efforts. He spent time undercover within the Empire as part of this endeavor. Upon learning about an Alderaanian young woman possessing the extraordinary ability to discern a person's true nature at a single glance, Nomen Karr promptly claimed her for the Order and designated her as his student. He devoted his life to her instruction and protection, hoping that she could expose those he perceived as threats to the Galactic Republic or the Jedi Order. However, the Jedi High Council remained skeptical about the infallibility of Karr's Padawan's power. As part of her training, he dispatched his apprentice to Tatooine to receive instruction from the esteemed Jedi Master Yonlach.

During the Cold War, Karr discovered that Darth Baras, his longtime adversary, had acquired a remarkably powerful new apprentice. Continuing his surveillance of Baras's assets, Karr instructed a Republic orbital monitoring station in Hutt Space to monitor and track the movements of the apprentice's ship. The apprentice was en route to Balmorra and Nar Shaddaa, which Karr believed were providing extraction for Baras's agents.
Upon discovering the surveillance, Baras and his apprentice dispatched the latter to the listening station, where they violently eliminated the stationed spy. Karr, observing the station via hologram, implored for the lives of Hirosho and the other technicians. The Sith disregarded his plea and slaughtered them before Karr's eyes. Appalled by what he had witnessed, Karr declared the Warrior to be a true minion of Baras. After regaining his composure, he informed the apprentice that he was unaware of Karr's plans. However, the Sith surprised him once more by revealing their knowledge of his Padawan and dismissing her powers as a sham. Briefly disconcerted by the revelation, Karr vowed to protect his Padawan from the taint of the Sith, even as the Warrior promised "to keep her skull as a trophy." Denouncing the Sith as unreasonable, Karr summoned a squad of elite troopers, proclaiming them to be the Republic's finest, and demanded the Sith's surrender, which was refused. The apprentice swiftly annihilated the attack squad. Recognizing the Warrior's ferocity, Karr stated that he had gained insights for their eventual confrontation.
The listening post incident played a crucial role in Karr's downfall. Recognizing Karr's protectiveness of his Padawan, Baras instructed his apprentice to locate and eliminate those closest to her, beginning with Yonlach on Tatooine and eventually her parents on Alderaan. Baras's intention was to destabilize Karr's apprentice and lure her out so that she could be killed.
The plan ultimately succeeded, and Jaesa contacted Baras's apprentice to arrange a private meeting. Karr soon learned of her arrangement and denounced it as foolish and rash. He forbade her from attending the meeting and instead dispatched two trained Jedi Knights, Ulldin and Zylixx, to eliminate the threat permanently. However, the pair proved no match for Baras's apprentice and were swiftly defeated.
Growing weary of the endless intrigues, Karr contacted Baras directly to arrange a duel to finally resolve their conflict. The stage was set for a final battle at the site of Karr's betrayal years earlier. However, fearing a trap and aware of his apprentice's immense strength, Baras sent the Warrior in his place. Karr was deeply disappointed to find the Warrior alive and in Baras's stead but remained confident that he could defeat the apprentice and expose Baras's network of spies. However, the Warrior proved to be stronger than he anticipated, both physically and in the ways of the dark side.

Karr succumbed to the Warrior's taunts and embraced the dark side to gain the strength needed to win their fight. The Jedi Master failed and was defeated by Baras's agent. Mortally wounded, Karr felt that the dark side had underestimated him and began to fight his way back to the light, willing to die to protect Willsaam. However, Imperial operatives dispatched by Baras managed to stabilize him so that the Warrior could use him as bait to lure Willsaam into the trap. In a moment of true darkness, the Wrath chose to torture Karr, promising that he would "suffer as no one has ever suffered." The ruse was successful, and Jaesa arrived to witness her master twisted by the dark side into something unrecognizable, spewing his hatred for both Baras and his apprentice. Initially, she considered the possibility that Karr had been hiding this all along before accusing the Sith of corrupting her Master. The Warrior told her that the only way to find the truth was to use her power on him. Willsaam complied and saw these dark traits within Karr but refused to believe it. The Warrior continued to taunt her, urging her to give in to her anger and lash out. She engaged the Sith Warrior in fair combat but proved no match for the Sith apprentice. Surrendering herself and embracing the path of the dark side, Jaesa became the Warrior's new apprentice. Karr fought against the dark side within him and revealed a part of himself in the light.
He attempted to convey to his apprentice that she was not adequately trained to confront a Sith, but Willsaam was drawn to the Sith Warrior's path, attracted to this newfound power—using brutality to uncover the truth—and, as the Warrior pointed out, Karr himself was a master of the light but had failed as much as she had.
As Willsaam pondered what to do with her former master, the Warrior ultimately instructed her to kill him as her first test to become strong in the dark side. Regretting his role in his apprentice's corruption and allowing himself to become a slave to the dark side, Karr pleaded with her not to become a murderer, but Willsaam retorted that the Jedi were hypocrites for taking lives themselves and immediately decapitated and killed her former master.
As a Jedi Master, Nomen Karr exhibited unwavering dedication to the light side of the Force. This commitment drove him to expose Imperial infiltrators hidden throughout the galaxy, despite Baras's constant interference. Despite his affinity for the light, he was not opposed to using the dark side. He believed that the dark side, while abhorrent, could be employed when the situation demanded it. However, he arrogantly believed in the light's superiority over the dark side, displaying a darker sense of pride.
Beneath his noble and dispassionate demeanor lay a concealed dark persona. Karr assumed that he had left this dark reflection behind after infiltrating the Sith, but it was later brought to the surface by the Emperor's Wrath. Karr momentarily succumbed to his anger and frustration, believing that it would help him defeat his enemy and save his padawan, for whom he cared deeply. In doing so, he contradicted the Jedi teachings. With the dark side clouding his mind, he began to unleash all his repressed hatred for Baras through his apprentice.
In the final moments of his life, however, Karr began to feel remorse for his errors. Demonstrating remarkable willpower, the Jedi Master managed to fight his way back toward the Light, urging his apprentice not to succumb to the darkness. Despite this plea, his former apprentice succumbed to the dark side and became a Sith.
Nomen Karr was a master of lightsaber combat. He excelled in Soresu, demonstrating the ability to catch, absorb, and deflect numerous attacks from the Sith Warrior. Upon turning to the dark side for increased strength and power, he adopted a more aggressive approach, employing Form IV, at which he was also exceptionally skilled. Karr could seamlessly transition between both forms at will. Using the Force, he could leap back and forth with great speed between two opponents and hurl a lightsaber. Occasionally, he would use Force push to create distance between himself and an adversary before re-engaging in the duel. Karr was also highly proficient in utilizing Tutaminis. Possessing such abilities, he was a formidable opponent.
When he succumbed to the dark side during his duel with the Sith Warrior, Karr employed techniques such as Force choke, lightning shield, and Force rage. These abilities significantly enhanced the Jedi Master's already considerable strengths, making him an even more lethal adversary. Karr's Force rage allowed him to gain a substantial burst of energy for a brief period. Unlike other Jedi who used the same technique, Karr could somewhat control its power, and despite resorting to the dark side, he was not entirely consumed by it. This was due to his prior experience operating behind Sith lines, during which he also harnessed the power of the darkness but ultimately remained steadfast in his beliefs.
In The Old Republic, players who choose the dark side dialogue options can be the ones to kill Nomen Karr, as Baras desired his death rather than allowing Jaesa to kill him. Alternatively, players can send Karr to Dromund Kaas and allow Baras to determine his fate, which greatly pleases the Sith Lord.
Michael Gregory provides the voice for Nomen Karr in Star Wars: The Old Republic. He also voices Harron Tavus.

If players opt for the light side dialogue options, Karr's responses will differ from those in the dark side story.
As Karr observes the Sith Warrior sparing Hirosho and the other technicians at the listening station via hologram, he is taken aback, remarking that "your temperance is remarkable." However, this does not prevent him from deploying the Republic soldiers against the Warrior. After the Warrior defeats them, the Sith attempts to persuade Karr that they can resolve the situation peacefully. Karr acknowledges that he wishes it were possible before terminating the transmission.
At Karr's safe house on Hutta, the Warrior attempts once more to explain that they have not come to fight Karr, but the Jedi Master refuses to listen.
During the initial phase of the fight, Karr delves deeper into the dark side. The Warrior attempts to persuade him to surrender, but Karr refuses and continues to attack.
Soon, Karr sustains serious injuries and prefers to die, preventing anyone from reaching Jaesa. The Warrior attempts to convince him that they are not there to kill Jaesa, but Karr dismisses this as a lie before losing consciousness. Imperial troops arrive and restore him to health.
After Karr awakens, shackled to a chair, he assumes that the Warrior spared him so that Jaesa would believe they were "pure," which is true in the case of Light Side Sith. When Jaesa arrives and sees Karr in his maddened state, the Warrior instructs her to use her power on Karr to see what he truly is. Reluctantly, Jaesa complies and discovers "pride and... envy and hate... and vengeance" within her Master. Karr argues that it is all a trick and urges Jaesa to use the power on the Warrior, to which the Sith agrees. Jaesa sees "mercy... and fairness and... even perhaps compassion." She now understands this because the Warrior had previously spared Master Yonlach on Tatooine and her parents on Alderaan. Karr remains unconvinced, claiming that the Sith is attempting to disguise both of them so that Jaesa will "doubt" her powers. Jaesa is confused, believing that both Karr and the Warrior are trying to drive her mad. Karr blames Darth Baras's manipulation and orders Jaesa to kill the Sith Warrior so that she can see. The Warrior has no intention of fighting, but a bewildered Jaesa activates her lightsaber and tells her opponent to raise their defenses.
Jaesa is defeated, and the Warrior refrains from delivering the killing blow. Jaesa now recognizes that the Warrior's sense of darkness is a "mask" and that Karr holds a "much... uglier one." The Warrior assures her that the Jedi Master's Force connection is "flawed." After enduring numerous lies, including those from the Jedi, Jaesa sees that the Warrior possesses the power to reveal someone's "true nature." She agrees to become the Warrior's Sith apprentice, but in a lighter sense of the term.
Karr is detained by the Imperial soldiers, either to Baras, left to his "misery," or to the Jedi Temple on Tython to atone for his actions. Enraged, he will witness Jaesa being disavowed and labeled an enemy of the Jedi Order, but she feels that he "took me as far as you were able." He is led away, ranting.
On the Fury-class Imperial interceptor, if Karr is left alone or taken to Tython, the Warrior has the option to lie to Baras, claiming that the Jedi is dead, which earns affection points from Jaesa and Vette.
Even if he is taken to Tython for healing, his former apprentice, Somminick Timmns, states that his emotions have since been stunted and that he no longer communes with the Force, suggesting that he has experienced a crisis of faith.
Instead of urging her to fall to the Dark side, the Sith warrior urged her to stay true to her ideals and bring balance to the Force.
As Willsaam was concerned of what to do with her former master, the Warrior prompted that do she dispose of Karr or that they hand him over to Darth Baras. Initially hesitant at the prospect of him being turned to the Dark side, Willsaam instead watched her former Master be struck down by her new master, the Sith Warrior. With Willsaam now an agent of the Sith, but still holding on to her Jedi training and moral, she became a force for reforming the Empire.