Ben first transforms into Echo-Echo in Ben 10 Returns Part 2.
Echo-Echo can emit ultrasonic-frequency sound blasts from his mouth powerful enough to disrupt machinery. Echo-Echo's other ability is to duplicate his being and thus can launch multiple sonic blasts from different directions. Using these two in tandem, Echo-Echo can potentially crush any foe with sonic blasts from all directions by surrounding them. Due to his small size, Echo-Echo can bypass larger foes with ease.
Echo Echo's strengths are duplication, sound manipulation, and small size, which seem to come from Ditto (duplication), Benwolf (sonic howl), and Greymatter (small size). Since he can duplicate numerous times with no limit, This can be used as a prime key in defeating enemies. e.g surrounding enemies or larger foes. Echo Echo is also a little bigger than Greymatter. Greymatter is the size of your palm, while Echo Echo as high as your knee. Unlike Ditto, when one gets hurt, none of the others feel the same pain. Echo Echo is also not as sassy as Ditto.
Because of Echo-Echo's small size he can often be in danger of being trampled if in a crowded area, but he makes up for this with his duplication abilities.