Rex Bactapack was born and cloned on the water world of Kamino, where he was trained and befriended by many of his fellow brothers. Two of his most beloved friends were CT-1113, Brion Graulshrieker, and CT-2255, Captain Xhal, who was in his squad with him on Kamino. Rex and his squad were subjected to physical and mental training on the standard "Citadel Court." Him and his squadmates worked together as a team and accomplished the challenge with ease. Upon testing day, Rex and his team completed the course with ease, just as if they were taking the practice test, which impressed the officer overseeing the test, which was Captain Rex, leader of the famed 501st Legion. Immediately after graduation, Captain Rex wanted Bravo Squadron to join the 501st Legion. All of Bravo squad were very pleased with this, and joined the 501st down to the planet surface of Geonosis.