Kueller, originally known as Dolph, was a Human male possessing Force-sensitivity. Prior to 17 ABY, he was a student at Luke Skywalker's Jedi Praxeum situated on Yavin 4. Early in his training, Dolph's homeworld, Almania, was subjected to a purge by the ruling Je'har government, which aimed to suppress all resistance, including Dolph's parents. The intense pain and sorrow stemming from their deaths led him to abandon the academy and embrace the dark side of the Force. Renaming himself Kueller, he initiated a resistance movement with the goal of eradicating the Je'har. Harnessing the power of the dark side, he successfully achieved his objective by utterly destroying the Je'har and their supporters. He also attracted Brakiss, another of Skywalker's unsuccessful students, to become his apprentice. As part of his actions, he annihilated the population of Pydyr, one of Almania's moons.
Despite the Je'har's defeat, Kueller harbored resentment towards Skywalker and the New Republic for their inaction in aiding Almania during the Je'har's attacks. He assigned Brakiss to oversee a droid manufacturing facility on Telti, where a large number of explosive-laden droids were produced for sale to the New Republic, including many used in their fleet of X-wing starfighters. When Kueller activated some of these droids, a series of explosions tore through the Senate Hall on Coruscant, resulting in injuries to Chief of State Leia Organa Solo and causing widespread disruption within the New Republic as part of his scheme. In response, Skywalker pursued his former student to Almania, but Kueller sabotaged the Jedi Master's X-wing. Following a crash landing, the wounded Skywalker was easily captured, but he managed to escape. Simultaneously, Organa Solo led a New Republic fleet, commanded by General Wedge Antilles, to confront Kueller. The Dark Jedi ordered his forces to engage the New Republic fleet while he dealt with Skywalker.
Upon catching up with Skywalker on Almania's surface, Kueller defeated the weakened Jedi Master and threatened to kill his recently arrived sister, Organa Solo. However, the arrival of Han Solo and Mara Jade, along with a ysalamir, shifted the balance. The Force-repelling effect of the ysalamir and the arrival of Skywalker's allies undermined Kueller's advantage. Enraged, he prepared to activate another wave of detonator-equipped droids, but C-3PO and R2-D2, located on Telti, deactivated his bombs. Leia Organa Solo then fatally shot him, putting an end to his plans for a new rebellion.

On the planet Almania, Kueller was born as Dolph, and it was discovered that he had Force-sensitivity. Subsequently, he began training to become a Jedi Knight under Jedi Master Luke Skywalker at his Jedi Praxeum on Yavin 4, where he was regarded as one of the more gifted students. However, he possessed an element of internal darkness. After less than a year of training at the Jedi Academy, Dolph received the news that his parents had been murdered by the Je'har regime, the government in power on Almania at that time. Upon leaving the Academy to travel to Almania to verify the news, he discovered their bodies impaled in front of the governmental palace, having been killed by the Je'har.
Dolph was horrified and enraged by the atrocities he witnessed, leading him to succumb to the dark side of the Force and develop bitterness toward the New Republic, particularly its Chief of State, Leia Organa Solo, who, due to legal limitations, did not intervene against the Je'har despite being urged to take action. Seeking vengeance against the government that had killed his parents, he utilized his powers to assist in overthrowing the Je'har regime and gathered a following of supporters, operating from the world of Fwatna. During this time, he concealed his identity with a Hendanyn death mask and adopted the name Kueller: a notorious, long-deceased Almanian general. The ranks of his forces rapidly grew, and he led them to victory, destroying the Je'har regime and killing all those he labeled as sympathizers of the old regime. Declaring himself the Master of Almania, he relocated his headquarters to the Great Dome of the Je'har and began planning his next course of action.
After several months, Kueller selected his next target: the New Republic, which he held responsible for the deaths of his family and people at the hands of the Je'har. He also encountered another errant apprentice from Skywalker's academy, Brakiss, who had been an Imperial spy sent to infiltrate the Jedi Praxeum. However, Skywalker had forced Brakiss to confront the darkness within him, causing Brakiss to flee, emotionally conflicted. When Kueller found him, the Almanian leader assisted in repairing Brakiss's damaged psyche and integrated him into his plans.

Kueller devised a plan to undermine the New Republic by planting explosives in droids and critical components of computers manufactured under his direction. He accomplished this by dispatching Brakiss to manage a droid factory on Telti, where the specially rigged droids would be produced, and Brakiss was pleased with his assignment. Brakiss's role as supervisor would prevent anyone outside of the plot from discovering the bombs.
The droids were sold at discounted prices to specific buyers, allowing Kueller to use the bombs as a scalpel to erode the faith of a populace that had already endured crisis after crisis under New Republic rule. He used the revenue generated from the sale of the droids to acquire all available Imperial surpluses, intending to capitalize on the fear a resurgent Empire would provoke and further destabilize the Republic. To achieve these goals, he employed smugglers such as Jarril to obtain his military surplus items and distribute some of the droids. His plan proved successful, as a large number of the droids and computers ended up in the hands of the New Republic government and New Republic Defense Force. In particular, the New Republic's latest model of T-65 X-wing starfighter was equipped with a computer sold by Kueller at a bargain price and containing a detonator, designed to eliminate a significant portion of the New Republic's starfighter pilots when activated. Furthermore, he also sold droids to the wealthy inhabitants of Almania's moon of Pydyr, and some were distributed to Almania's other moon of Auyemesh as well. This infiltration occurred over a period of two years as the droids were sent to infiltrate the New Republic and the two moons of Almania. Eventually, he exterminated the entire population of Pydyr by activating the droids, with the exception of the seafah jewelers, as their skills in extracting seafah jewels were indispensable. By plundering the considerable wealth of Pydyr, Kueller amassed a large reserve of credits and became Almania's undisputed ruler.
Kueller thrived on each death he caused to gradually enhance his power in the dark side, which would enable him to defeat the Skywalkers, thus becoming a new Emperor, as he was not satisfied with merely ruling the Almanian system. He also enjoyed tormenting his subordinates, primarily Brakiss and a woman named Femon, although he allowed Brakiss to return to Telti after his apprentice provided him with the remote that controlled the detonation circuits of the sabotaged droids distributed throughout the galaxy, particularly to the New Republic government.
He then decided that it was time to directly attack the New Republic. The next phase of his plan involved detonating the rigged droids in the New Republic Senate, self-destructing them as its first session was being opened. This resulted in the deaths of representatives from several planets, forcing them to hold emergency elections. Several more former Imperial officials became representatives of their planets, giving the opposition to Chief of State Leia Organa Solo a simple majority in the Senate. When her husband, Han Solo, became suspected of involvement in the bombing, her response prompted the Senate to begin calling for a Vote of No Confidence. Furthermore, Kueller planted evidence to help raise suspicions about Han Solo.
After Pydyr was declared largely purged of inhabitants and cleared of their remains thanks to his droids, Kueller visited the moon and was pleased with the precise destruction he had wrought, as well as the wealth he had now accumulated. He spent several days there, meeting with his spies and cataloging the matériel he had acquired, but during this time, Femon plotted against him. Upon his return to Almania, she declared that he was too consumed by revenge and decided to kill him. Her betrayal surprised Kueller, but he was prepared for it nonetheless, executing Femon before she could activate any of her traps, including one that would suffocate everyone in the room while she was whisked down a passage. Kueller would later miss her intelligence, feeling that his subordinates were not her equal.
Kueller then set several traps to lure Luke Skywalker to Almania. He left one trail of smugglers intact, hoping to lure Han Solo or New Republic ally Lando Calrissian to Almania to use as bait for Skywalker; to that end, he hired Ana Blue. He also ordered Brakiss that, should Skywalker arrive on Telti, he was to either kill the Jedi or send him to Almania. Brakiss did eventually confront Skywalker on Telti, and the Jedi Master was informed that Almania was where the source of the disturbances in the Force caused by the deaths of Pydyr's inhabitants had originated. Kueller received Brakiss's report that Skywalker was en route to Almania and congratulated his apprentice on a job well done, leaving him on Telti, as Brakiss wished.
Skywalker traveled to Almania and decided to land on Pydyr, but his X-wing contained a computer with a built-in detonator. During his landing, Skywalker accidentally triggered the detonator, and his ship exploded. Although he survived the crash, he was severely injured. After allowing the Jedi to wander through deserted Pydyr for some time, Kueller confronted him. To his surprise, Skywalker identified him as Dolph, but that did not deter Kueller. Each attempted to persuade the other to join him, but the two men rejected each other's offers, leading Kueller to draw his lightsaber and attack Skywalker. After a brief duel, Kueller overcame the injured Skywalker and had him imprisoned, hoping to use him as bait for Leia Organa Solo, Skywalker's sister.
Kueller further undermined Organa Solo by demanding that she allow him to become the ruler of the New Republic, or else her brother would be executed, showing her surveillance footage of a badly injured Skywalker. He also activated another round of sabotaged droids on Auyemesh, causing over a million deaths as a demonstration of his powers, which the Force-sensitive Organa Solo felt. Instead of giving into the Dark Jedi's demands and allowing the no-confidence petition to come to a vote, she resigned from the position of Chief of State. Mon Mothma, Organa Solo's predecessor, took over for her.

Organa Solo departed for Almania to find her brother, accompanied by a small fleet led by General Wedge Antilles. Upon their arrival at Almania, Kueller was in his command center, prepared for them with three Victory-class Star Destroyers crewed entirely by droids. He activated more sabotaged droids, hoping to cripple the New Republic fleet, but some of the droids had been stolen by smugglers and detonated elsewhere. Additionally, the New Republic had already detected the sabotage and had simply deactivated many of their droids and X-wing fighters. Placing the fleet under the command of his subordinate, Yanne, Kueller ordered him not to fail. Sensing that Yanne was losing enthusiasm for his cause, Kueller gave the man a verbal and physical warning of the consequences of disloyalty. After Yanne was dismissed to conduct the battle, Kueller had another subordinate, Gant, brought to him, thinking he might need to train another personal assistant to replace Yanne. However, despite the efforts of Kueller's fleet, Organa Solo managed to slip past them and their fighters and made it to the surface of Almania, which pleased Kueller, who anticipated destroying both Skywalkers at once. When informed of the arrival of the Millennium Falcon and Wild Karrde—belonging to Han Solo and smuggling kingpin Talon Karrde, respectively—he ordered Yanne to destroy them, but his subordinate explained that he needed to finish off the New Republic fleet first, an explanation Kueller accepted.
Kueller's fleet was initially successful, inflicting heavy losses on the New Republic task force. However, by observing the actions of Kueller's fleet closely, General Antilles realized the true nature of the crew of the Star Destroyers and their fighters. He ordered his gunners to fire on the other ships in the Republic convoy. This caused the droids in Kueller's fleet to believe that the ships of the Republic fleet were actually on their side. The Star Destroyers went into a flanking position to protect their "allies." They were then crippled by precise turbolaser fire from the Republic fleet.
Back on Almania, Skywalker escaped from Kueller's underground prison with the aid of a thernbee, a creature that was supposed to kill him but whom Skywalker had befriended, and made his way back to the surface of a Pydyr city. Having arrived to save her brother, Organa Solo found Skywalker, and both made their way to her ship. Skywalker was still weak, and Organa Solo accidentally left him behind when she rushed ahead to open the ship. In those moments when they were apart, Kueller emerged from his hidden command center, found the Jedi Master, and forced a lightsaber duel between them. Organa Solo came back to find both locked in combat. As they dueled, Skywalker abruptly decided to give up, realizing that Kueller was feeding off both the deaths that were occurring across the galaxy from his most recent activation of detonator equipped droids, which had actually been shipped to the Smuggler's Run, and Skywalker's own anger. As Kueller raised his lightsaber to strike Skywalker down, he lost his connection to the Force. He tried to use his anger to tap back into the Force, but doing so was useless, since a thernbee that had swallowed an ysalamir nutrient frame had wandered into the general vicinity. The ysalamir, which generated a Force-empty bubble, had been brought by a New Republic rescue party made up of Han Solo, Chewbacca, and Mara Jade, who had followed the smugglers' information to Almania. Kueller had one last trick up his sleeve, though, and he used a remote to send the signal to self-destruct his remaining droids. However, Kueller's last act of malice was thwarted thanks to the efforts of R2-D2, C-3PO, and a young mechanic named Cole Fardreamer, who had gone to Telti and disabled the detonators. As Kueller had attempted to activate his remaining bombs, Organa Solo shot him twice, and at least one blast hit him in the head, killing him. She removed the death mask from his lifeless body, revealing Dolph's boyish face.

Kueller's bombs inflicted damage on the Grand Convocation Chamber of the Senate Hall that led to its complete replacement, a project finished in 19 ABY. Although Kueller's bombs destroyed the Senate Hall, they caused little damage to the building's actual structure, allowing the Grand Convocation Chamber to be built on top of the original structure. Three years after his death, a vision of Kueller appeared to Mara Jade in a nightmare shortly before her wedding to Luke Skywalker, the fallen Jedi appearing among other dark side figures such as the Sith Lords Darth Vader and Emperor Palpatine. Kueller's apprentice, Brakiss, later became the leader of the Shadow Academy and led the Second Imperium in a brief conflict with the New Jedi Order, but he was ultimately defeated. Kueller's henchman, Yanne, later took over the Almanian government, and the planet later developed industrial connections with Tendrando Arms by 40 ABY.

Throughout most of his adult life, Kueller was aligned with the dark side of the Force. Luke Skywalker described him as deeply wounded and having experienced great pain, but Kueller attributed much of his strength to the anger he had cultivated over the years. He harbored a deep hatred for the Je'har and the New Republic, the former for murdering his countrymen and parents and the latter for failing to oppose the Je'har, despite having the power to do so. He was also ambitious, desiring to completely rule the galaxy. However, despite his ambition, he understood the value of patience, waiting until what he deemed was the appropriate moment to unleash his plan for a galactic rebellion instead of rushing his plans, as his henchwoman, Femon, desired. As such, he made sure to thoroughly check each stage of the preparations himself, as he distrusted many of his subordinates. He was also aware of his own weaknesses, knowing he could not face powerful Jedi such as Skywalker face-to-face without an advantage.
In his dealings with subordinates, Kueller tolerated no arguments and was known to execute and threaten those who failed or disobeyed him. He felt some camaraderie with Femon, but her later insubordination led him to choke her to death with the Force. Kueller believed that only those who understood the power of the Force could truly control the galaxy, and he used his own Force power to influence and manipulate others. He killed countless beings across the galaxy in his bid for power, ending the lives of at least a million in one instance as a demonstration of his abilities to Leia Organa Solo. His apprentice, Brakiss, believed that Kueller never forgave anyone who crossed him. Kueller was also known to wear a white Hendanyn death mask, complete with black accents and set with tiny jewels in the corners of the eye slits, and a suit of armor, which he wore when appearing to most of his subordinates or adversaries, such as during his encounters with Luke Skywalker and his ultimatum to Chief of State Leia Organa Solo. This gave him a foreboding appearance that he used to awe and impress people and also served to disguise his identity.

During his time as a student at the Jedi Praxeum, Kueller was considered extremely powerful, but he left before his training could fully develop his abilities. As a Dark Jedi, Kueller wielded a blue lightsaber and was able to defeat Luke Skywalker in a duel, although his opponent had been badly injured in a starfighter crash. He also possessed the power of telekinesis and could choke people with the Force. When Leia Organa Solo fired upon him with a blaster, he easily deflected her fire and threatened to use the Force to make the weapon explode in her hand. Kueller was able to grow stronger and more powerful by absorbing the fear and anger of others, as well as the dark side disturbances caused by the deaths of countless individuals across the galaxy. Kueller also had knowledge of Force healing and battlemind, in addition to familiarity with computer systems and demolitions, and he was literate. Finally, he was skilled at manipulating others, from his apprentice Brakiss to high-ranking New Republic officials, playing on their weaknesses.
Kueller made his first appearance as the primary antagonist in Kristine Kathryn Rusch's novel, The New Rebellion. He later appeared in several reference works, but aside from mentions and being briefly pictured in a dream in Star Wars: Union, he never made any further appearances in Star Wars stories. The Essential Guide to Planets and Moons includes an illustration of Kueller with an unnamed human; this was later identified as Kueller without his armor in The Official Star Wars Fact File 93. Kueller's hair is described as silvery in the Star Wars Encyclopedia, whereas The New Essential Guide to Characters states that it is brown.