Emilia said in Episode 48, “Who is this? This should be Echidna’s grave…” ’cause she didn’t recognize the corpse, which was that of an older woman compared to the Echidna seen in the Sanctuary trials. Roswaal and Beatrice, however, recognized the corpse as their teacher/mother, showing that the Echidna in the Sanctuary trials and tea parties was not the same as the one Roswaal and Beatrice knew, showing that Echidna had previously cut off a piece of her soul when she was younger, which would explain the presence of the younger Echidna in the tea parties. Episode 50 confirmed this with a small animation of the older Echidna Roswaal and Beatrice knew whose face was closer to that of the corpse.
We know from Season 2 the Echidna from the tea parties had been able to place her soul into Ryuzu Shima, which is how Echidna spoke in her own voice after Emilia’s remark about saving the original Ryuzu Meyer got Echidna annoyed, which caused her to break her act momentarily, allowing for viewers to hear Echidna’s voice through Ryuzu Shima’s body, showing that Echidna’s immortality project had succeeded and that she created at least one soul clone, so it’s not out of the question that the Echidna Roswaal understood to be his teacher may also have been revived at one point. The Echidna Roswaal knew to be his teacher also may not have even been the original Echidna, but she is the one he long sought.
In Episode 48 of Re:Zero, one of the lines from the possible futures Emilia could face had Roswaal say, “I made the realization that I have not walked alone all this time. That is all.” For Roswaal, whose single goal that has kept him alive these past four hundred years is to bring back his teacher, it suggests that there’s an Echidna, not just any Echidna, but his original teacher, who is lurking near the Emilia camp, and if not the Emilia camp, then Roswaal himself, given he would never speak like that in reference to anyone else. What he spoke of suggests a physical closeness to Echidna that made him come upon a happy realization, and I wonder who in the cast could be her vessel or whether it’s someone not yet revealed.