What was Echo's flaw?

Answer 1

Echo's "flaw" was her unrequited love for Narcissus. She was not able to overcome his rejection of her. In the Greek myth, she pined away, eventually dying and leaving nothing but her voice, an echo.

You go go even further back and say that Echo's sin or flaw was that she was a gossip, and in being a gossip, told Hera something that she didn't wish to hear, which lead to her being punished to only repeat what others say and to never have her own voice again.