One of the NYMPHS, she was beautiful and had a most musical voice. She was employed by ZEUS to divert HERA’s attention away from his illicit love-making activities. When ZEUS slipped out for a night on the town, ECHO would deluge HERA’s ears with a constant stream of babble, gossip and gibbering until he got back.
"... So anyway, I said, I said to her, oooh, I said, I don’t believe it, and then she said to me, you won’t believe what he said to her, she said, and so I said..."
HERA, driven half-insane by the mindless wittering, removed her power of speech and kicked her out. All ECHO could do then was wander the countryside repeating the last syllable of the most recent person to give utterance.
After falling in love and being spurned by Narcissus, she took to living in caves where she had many a one-sided conversation. PAN was willing to overlook her affliction but she turned him down, so his shepherds tore her to pieces and scattered her all over the place. Now she’s practically everywhere and will talk to anyone who raises their voice.