Soul Echoes are beings and locations from past time loops appearing in the current timeline — flaws in the timeline. They had existed before the “Broken Clock” time loop, but were extremely rare until then. When the Expedition damaged the Anchor, time resets became increasingly flawed and the Soul Echoes began to manifest frequently.
People who are Soul Echoes appear not to be sentient, repeating the actions from their time loop. When anomalies caused by the time loop are pointed out, they appear confused — pushed too far and they will ignore the provocateur, moving through them like a ghost. However, some Soul Echoes are fully conscious. Notably, the Puppets from the “Death of Free Will” loop are actively protecting their territory and seem to have retained all memories from previous time loops since their creation.