In the pure world, killing mirror images is the only way to survive. The Shadow Killers are more about the outside world than themselves.
But Lechi never knew why he was fighting - the only thing he had was a plain ring around his helix and a mirror image that always spoke strange words: Fang Zhengdu.
Fang Zhengdu cannot be killed no matter what, and he never drops shadow cores. He will only say, "Remember me, Fang Zhengdu."
All Shadow Killers will have their memories wiped after completing their missions, but...
Le Chi clearly remembered Fang Zhengdu and his words.
Purpose: Improve memory