FALL OF THE REPUBLIC
Also known as CT-1409, Echo was a member of Domino Squad on Kamino before graduating and joining the fight against the Separatists. He survived a Separatist attack on the Rishi Moon listening post, was decorated for bravery, and inducted into the 501st Legion. Echo and his fellow Domino, Fives, later helped defend Kamino against a Separatist invasion, and were promoted to ARC troopers after the Separatists retreated. The two also helped Anakin Skywalker and Obi-Wan Kenobi free prisoners from the forbidding Separatist prison known as the Citadel, a mission that cost Echo his life -- or so his friends believed.
When Echo's voice was intercepted in a Separatist transmission, Captain Rex became convinced his friend was indeed alive. With Anakin Skywalker at his side, along with the Bad Batch, the elite squad infiltrated a Separatist base on Skako Minor and discovered Echo, or what was left of him, locked up amid a mess of tubes and wires. More machine than man now, Echo's body was patched into the Separatist computer network, his brain working out the complex calculations and battle strategies referred to as the algorithm while his injured and mutilated body remained in stasis. A prisoner of war, Echo was being forced into revealing the clones' combat strategies, giving the Separatists an advantage on the battlefield.
After his rescue, some were uneasy about Echo's abilities and, frankly, his loyalties considering his ordeal. In their experimentation, the Separatists had stripped away Echo's humanity and tried to turn him into a machine. Gaunt and mechanized, he was able to lead a strike team behind enemy lines to help secure the base at Anaxes and prove that he was still loyal to the Republic and his brothers. But his path now lay separate from his longtime friend Captain Rex.
THE DARK TIMES
At the end of the Clone Wars, Echo became the fifth member of Clone Force 99. He was serving with the Bad Batch when the Emperor executed Order 66, and the clone army turned against the Jedi they had served in the war. Back on Kamino, Clone Force 99 was tested by the Empire for compliance to the new regime.
After an Imperial mission to eliminate an insurgency of Separatist droids on Onderon -- only to find organic beings just trying to survive -- Echo and his brothers defected from the new Empire alongside a child called Omega, who was a mutated clone like the rest of the Batch. However, Crosshair remained behind to serve the Emperor and Admiral Tarkin.
Echo and Tech often worked together as the team's focus pivoted from executing Republic missions to surviving under the radar of the Imperial rulers. His cybernetic implants helped Echo to scope out potential threats, like a energy-consuming creature they encountered after crash landing on the Ordo Moon.
On Pantora, Echo donned his helmet and was mistaken for a droid, sold off to a street vendor for the Bad Batch could obtain some much-needed credits for supplies. After revealing his true identity to the other droid's in the merchant's service, the mechanical beings helped Clone Force 99 finish repairing their shuttle so they could escape.
Later, as the Bad Batch met up with Echo's old friend Clone Captain Rex to have their inhibitor chips removed in an old medical bay on Bracca, Echo was instrumental in helping Clone Force 99 escape from Crosshair and his Imperial drones, with his keen senses detecting the lurking scrappers who reported their location to the Empire.