I definitely think that Ralsei is Kris's original soul. There are no other objects in the Light world that make sense, IMO. I'm 100% sure someone else has already covered this in exhaustive detail, but I wanted to write my own outline of the theory.
Food for thought:
Kris loves their brother more than anyone, so it makes sense that a representation of Kris's soul would look similar to Asriel.
On that note, Kris, Susie, and Asgore all remark that Ralsei only looks superficially similar to Asriel. I believe this is an indication that Kris is starting to view Ralsei as a unique and valuable friend, rather than a substitute. This is naturally reflected in their soul, which means that Ralsei is going to continue looking less like Asriel Lite and more like his own person.
Trans Ralsei would be a fun way to demonstrate this, especially considering Asriel is implied to be more on the masculine side (what with the pizza-scented deodorant and all.) Galsei says "not only am I not a boy, I'm also not that SPECIFIC boy."
Ralsei himself tells Kris that the Soul embodies their willpower and compassion. Ralsei is the most compassionate party member in the first chapter, and he has demonstrated more and more willpower with each subsequent chapter.
It's obvious that Kris is NOT supposed to be walking around without a soul. It makes them weak and sickly. Therefore, we can infer that they must have had a soul of their own before the events of the game proper. At some point, they removed their previous soul and replaced it with us, the player. So, their original soul must still exist SOMEWHERE.
Ralsei knows an awful lot about the game's interface, and, more specifically, how the interface is meant to be navigated by a soul. Granted, Susie says in Ch 5 that she is also capable of seeing the menu options, but Ralsei clearly understands them on a deeper level.
On that note, Ralsei is obviously aware that Kris is being puppeteered by the player. That's why he's always sending us away to have a private discussion with Kris. I would go so far as to say that Ralsei knows the Soul does not belong to Kris at all, which is information that he would only know if he and Kris had a pre-existing relationship of some kind.
The heart? On his cloak??? Hello?????
This theory's potential for heartbreak (no pun intended) is off the charts. Kris can't live without a soul. Assuming that the player will eventually have to leave Kris's body, Kris can only survive if their original soul returns to them. Therefore, Ralsei may have known from the very beginning that his days are numbered. In all likelihood, when the player is no longer able to intervene, Ralsei will be the one to seal the last fountain - the fountain in Castle Town. Then, he'll return to Kris.
Adding to this, I'm 99% sure that Kris somehow combined their original soul with a crayon drawing of Asriel, which resulted in Ralsei. It would explain why their green crayon was missing.
Also might explain why Flowery refers to Asriel as an "impossibility" - a Darkner who can traverse different Dark Worlds without being petrified, all because he possesses a human soul.
















