CT-21-0408, nicknamed "Echo," as he was known for "echoing" the standard rules and procedures established for clones, was a clone trooper who served in the Grand Army of the Republic during theClone Wars. Early in his career, Echo lived within Tipoca City, thecapital city of the planet Kamino, and trained in its military complex. As a clone cadet, Echo was part of the squad Domino Squad, overseen by the Siniteen bounty hunter Bric. Echo stayed with Domino Squad until the unit passed their final test; the completion of the test allowed the members of Domino Squad to become battle-ready clone troopers. For his first off-world assignment, Echo stood guard at Rishi Station, a listening post located on the moon of Rishi, where he and other rookie clone troopers were stationed. When theConfederate General Grievous implemented an attack on the Rishi Station to capture it before proceeding to Kamino. His plan, however, was thwarted by Echo, fellow troopers Fives, Hevy, and clone officersCody and Rex. Echo, along with Fives, were rewarded with medals for their service on the Rishi moon and were inducted into the 501st Legion.