Echo

Echo, also known as CT-21-0408 was a clone trooper who served in the 501st Legion under Captain Rex and Jedi Anakin Skywalker. Initially, Echo was a member of Domino Squad. He received his nickname because during his training, he would repeat the orders given to the cadets.

Training
Echo was training to be a clone trooper with his fellow cadets Fives, Hevy, Droidbait and Cutup who were in Domino Squad. The members of Domino Squad had difficulty working together at first due to their different personalities and strategies. Echo had a tendency to repeat orders and follow procedures, which annoyed his squadmates. They failed their final test but had a chance to retake it and they finally passed.

Rishi Moon
Echo, as well as the other members of Domino Squad were later stationed at the Rishi Moon listening post under the command of Sergeant O'Niner. Commando droids attacked the station and Echo, with the other troopers fought them off. Droidbait was killed when the droids attacked. Cutup was later killed when he was eaten by a Rishi Eel. Rex commented to the remaining clones on them being "shinies" due to their shiny armor and then placed his hand on Echo's armor, leaving a handprint from the eel's blood. Later, Hevy died sacrificing himself to save the others, which destroyed the outpost. Echo and Fives were the only remaining members of Domino Squad, and received medals for their service and they became members of the 501st.

Phase I
Echo at first had Phase I armor which was completely white due to him being considered a shinie. After the battle at Rishi Moon, his armor had a blue handprint, which he kept on all versions of his armor afterward.

Phase II
With Echo's Phase II armor, he was an ARC trooper so he had a kama and a pauldron. He still had the blue handprint on his armor.