Echoes are remnants of shadowflux. Lady Zillah mentions them in the seventh book, Flashback. When Sophie and Fitz had echoes, they caused them immense pain. Fitz had one on his heart, so he felt pain whenever he is feeling strong emotions in his heart or emotional center. Sophie had one in her head, and one in her hand. She felt pain when her anger and fear take over, causing a "monster," metaphorically speaking, that makes it harder to calm her echoes. Everyone's echoes are different. They can take different amounts of time to go away for each person. The triggers can also differ, depending on the emotion being felt when the Shadowflux was created. For Sophie and Fitz, the echoes went away with Flori's song.