Galen Marek was a powerful Force-wielder born on Kashyyyk during the Dark Times to Jedi survivors Mallie and Kento Marek. His mother died defending her family from Trandoshan slavers, and his father was executed by Darth Vader after they were discovered. Marek himself was abducted by the Dark Lord and taken on as his secret apprentice. Given the code name Starkiller by Vader, Marek forgot his life with his parents, his earliest memories being his tutoring under Vader. The Sith Lord's brutal training regime forged Starkiller into a powerful Force-sensitive, and Starkiller was utilized by Vader as an assassin, dispatching the Dark Lord's rivals and enemies. To aid in his missions, he was provided with Cloaking device-equipped light transport, the Rogue Shadow, along with a pilot, and the personal holodroid, PROXY. PROXY became Starkiller's only constant companion and friend, despite the former's programming to continually attack him with lethal force, part of Darth Vader's training regime. The pilot position on the Rogue Shadow had a high turnover rate, as they were either killed in the field or executed by Vader.As Starkiller's training neared completion, Vader set him with his first test; to hunt down and kill the Jedi survivor Rahm Kota. He also acquainted himself with his new pilot, Captain Juno Eclipse. While Starkiller defeated Kota, he failed to kill him, though Starkiller concealed this from Vader. Dispatched on a variety of other Jedi hunting missions by the Dark Lord, Starkiller completed them with brutal efficiency, all the while growing closer to Juno Eclipse. Finally, Starkiller was summoned by Vader to fulfill his purpose; help Darth Vader assassinate Emperor Palpatine, Vader's own Sith Master. However, it was in truth a ruse; Palpatine had discovered Starkiller's existence and ordered Vader to execute him. Starkiller was impaled and spaced, though his body was collected by Vader's droids.
Starkiller was taken to Vader's science vessel, the Empirical, his body revived and rebuilt. During this period, Starkiller was known as Subject Zeta or Subject 1138. Awakening from six months of unconsciousness, he was given a new mission by Darth Vader; to create an alliance of Imperial dissidents and rebels to distract the Emperor, allowing Vader and Starkiller to catch him unawares. With PROXY's help, Starkiller escaped the Empirical, rescuing Juno Eclipse while at it, she having been branded a traitor due to her association with Starkiller. Starkiller kept Juno in the dark as to his reason for gathering the rebels, though she became a full participant in its completion. Their first action was to retrieve Rahm Kota, to obtain his contacts and connections. The now depressed Kota, having been blinded during his duel with Starkiller, didn't recognize him, and joined his entourage. As they went about created the Rebel Alliance, Starkiller became conflicted, growing close to his allies while becoming increasingly disillusioned with the Empire, and he began to consider earnestly leading the Rebels against the Empire, abandoning the Sith. However, before he could take any such action, Vader sprang his trap.
When the Rebel leaders were gathered on Corellia to officially declare rebellion against the Empire, Darth Vader arrived with the Imperial Military at his heel and captured all the major players. He revealed to Starkiller that his entire mission had been a sham; nothing more than a tactic to gather all the rebels in one place so as to bring them down quickly. Vader also revealed that he had no intention of using Starkiller to assassinate the Emperor, and moved to execute him. PROXY stepped in and attacked Vader, allowing his master to escape before being disabled. With this final betrayal, Starkiller was finished with the Sith and the Empire. Retaking his birth name, Galen Marek decided to rescue the rebel leaders. Using the Force to track them to the incomplete Death Star I, Galen and Juno kissed for the first time before he leapt into the superstructure. Battling his way to the Emperor's observation dome where the rebels were being held, Galen engaged Darth Vader in a Lightsaber duel, defeating the Sith Lord. Palpatine gleefully offered Marek the chance to execute Vader and replace him as Palpatine's apprentice. While Marek struggled with the decision, Kota escaped from the guards, stole Palpatine's lightsaber and attacked the Emperor, though Palpatine easily fended him off. However, this prompted Marek to attack Palpatine. Marek refused Palpatine's command to execute him, and give in to his own darker nature, instead sacrificing his life to save the rebels, who were extracted by Juno.
Galen Marek died in a massive explosion of Force energy brought on by his battle with Palpatine. The severely injured Vader and Palpatine found his body, lamenting that his sacrifice had made him a martyr to the Rebel Alliance. In honor of his sacrifice, the Alliance took on Marek's family crest as their emblem. Darth Vader, now aware of Palpatine's desire to replace him with a more able apprentice, collected Galen Marek's body and cloned him, creating numerous aberrant monstrosities and two stable clones, Starkiller and the Dark Apprentice.