In Dollhouse, Echo (Eliza Dushku's character) is most notable for how she gradually became a full person. She developed a cohesive identity despite being in doll-state and was able to store the personalities she was imprinted with them in corners of her mind for later access, at times of her choosing. Towards the end of the show, as Boyd's true identity was revealed, he said that Echo was going to be the key to the developing an immunization to being wiped.
But was there ever any valuable information about why Echo? Any moments in the show, even fleeting ones, which offered any insight?