X-5 was a male Human assassin. He was one of seven men hired by Commander Rezi Soresh to take part in Project Omega, a Galactic Empire experiment aimed at engineering the ultimate assassin. Soresh erased the participants' identities from a database system, performed surgery to alter their appearances, and began a reconditioning program intended to erase their memories and emotions, transforming them into obedient servants.
Initially, X-5 formed bonds of friendship with the other trainees: X-1, X-2, X-3, X-4, X-6, and X-7. However, as Soresh subjected them to electrocution, X-5, along with the others, underwent a transformation of their mind and personality, losing all emotional capacity. Around 10 BBY, the trainees were forced to fight each other until only one survived. X-5 experienced a breakdown, whispering about forming alliances and urging the others to recognize Soresh as their true adversary. Before X-7 completely lost his emotions, he killed X-5, thus earning the position of Soresh's assassin.
X-5 was featured in a flashback scene in Rebel Force: Target, the initial book in the Star Wars: Rebel Force series for young readers. The book was authored by Alex Wheeler and released in 2008.