Warning I make a speculative guess that I do not elaborate on, but I do mention a name that does not appear until a little later on and I mention a concept that is brought up later on than that (just before the route split) so minor name spoiler and a spoiler on wishes.
Why do I have the strangest suspicion that the remnant is what Echo becomes and loses pieces of himself to on failed routes. I have a lot of feelings and thoughts that feed into this idea but I will not elaborate on them currently cause really a lot can change in the routes in time.
Edit: and yes this is after most of Sissel's route that is currently in the game