Galen Marek was a human male taken as a Sith Apprentice by Darth Vader, his father's assassin, a Jedi Knight named Kento Marek, on Kashyyyk during the Great Jedi Purge. For years, the Sith Lord taught him, under the code name Starkiller, the dark arts and how to eliminate his enemies, including the Jedi who survived Order 66. Marek was told that he would ultimately assist in overthrowing the master himself. of Vader, Darth Sidious, Emperor of the Galactic Empire. In reality, Vader and Palpatine planned only to use it to root out those who rebelled against the Empire. In 3 BBY, fifteen years after Marek's training began, he had begun his missions to search for the remaining Jedi, leaving no witnesses behind.

After killing Shaak Ti on Felucia, he returned to the Executor, his master's flagship, in which Darth Sidious appeared in holographic form and ordered Vader to kill the apprentice. In apparent submission, Vader telekinetically slammed him several times into the walls, then hurled him through the windows and into open space. Vader secretly recovered his body and had him "resurrected" six months later on the science ship Empirical. There, he was known as Subject Zeta. The dark lord convinced Starkiller that he had saved him from death against the orders of his Master (Darth Sidious), and gave him a new mission: to assemble an alliance of rebels who would attempt to eventually destroy the Empire.

After locating key figures, Marek and the rebels met on Corellia where they formed the Corellian Treaty, an agreement that created the Alliance to Restore the Republic and overthrow the Empire ruled by Emperor Darth Sidious. However, Vader, along with Imperial forces, attacked the gathering, capturing the rebels, noticing Marek force-pushing him into an abyss in which he grabs onto and is seen proxying with the obi-hologram. Wan Kenobi attacks Vader, which ends up beating Darth Vader by piercing him with a red lightsaber and revealing to Starkiller that they had only used it to gather the enemies of the Empire. and tell him that he would not overthrow the emperor with him. Marek fell off and was picked up by Juno Eclipse and followed the dark lord and Alliance leaders to the Death Star, a massive super-weapon in which he dueled Vader and Sidious, in which Galen sacrifices himself to that the Rebels manage to escape from there and form the Rebel Alliance