X-6 was a male Human assassin who was recruited, along with a group of six others, to take part in Project Omega. Commander Rezi Soresh initiated this project as an experiment with the goal of engineering the ultimate assassin, intended to serve the interests of the Galactic Empire. Soresh erased the identities of the participants from a computer database and performed surgical alterations on their faces before commencing a harsh reconditioning process, aimed at transforming them into obedient, thoughtless agents by erasing their memories and suppressing their emotions.
Initially, X-6 formed bonds of friendship with the other trainees in the program, namely X-1, X-2, X-3, X-4, X-5, and X-7. However, following the commencement of torture by Soresh using electrocution, X-6, along with the others, underwent a total transformation of their personality and mind, leading to the complete suppression of all emotions. Around 10 BBY, the aspiring assassins were forced to compete against each other in a battle to the death, until only a single survivor remained. X-7 swiftly killed X-6, thus earning the privilege of becoming Soresh's personal assassin.
X-6 made an appearance in a flashback within Rebel Force: Target, the inaugural book in the Star Wars: Rebel Force series for young readers, released in 2008 and authored by Alex Wheeler.