May I add some crazy theories of my own? Here's one:

My (strange) Theory:
Echo is in a sense a darker side of Naya who is attempting to control her to push itself to the surface. Echo is likely responsible for her memory wipe (if my theory is true, which it most likely isn't). So basically, my theory is that Naya has two souls inside of her- Herself, and Echo, each fighting for dominance and control over her mind and body- sort of like split personalities. But rather than like physical fighting (wait... how would split personalities physically fight anyway XD), I believe Echo is tricking her into its bidding (I don't know Echo's gender, sorry)

just read over my theory and realized how crazy it sounds XD