Rex was the second son born to Rafael and Violeta Salazar in Geneva, Switzerland, where he lived with them and his older brother Caesar. Rafael, Violeta, and Caesar were brilliant scientists who had traveled all over the world before Rex's birth. As a young child, Rex spent every summer in a small South American town, where he played with his best friend, Federico.

Prior to the Nanite Event, and during the Nanite Project which his parents and brother worked on, Rex was critically injured in an industrial accident caused by a dispute between the many scientists about how the naniteswere to be used. The scientists—among them Violeta, Rafael, Caesar, and Gabriel Rylander—injected Rex with the original batch of fully programed nanites, hoping to save Rex's life and to make the other scientists see that their ways were for the greater good. It was a major success as Rex was able to be saved. However, Rex's new abilities, courtesy of the nanites, only reinforced their beliefs. While Rex, Gabriel, and Caesar were able to survive the Nanite Event by blind luck, Rafael and Violeta were killed as a result of the massive nanite explosion.

Later, after losing his memories, assumed to have been caused by the explosion, Rex traveled to Hong Kong and befriended three incurable EVOs named TuckSkwydd, and Cricket. Unbeknown to them, Rex was also top lieutenant to an EVO crime lord named Quarry. During their time together, Rex and his gang had an encounter with the shape shifter John Scarecrowand all got voice activated nano-ink tattoos to prevent a repeat of the events. He later secretly traded his friends to Quarry in exchange for his own freedom.