Kris is the SOUL - clarifications and FAQ
It seems the reading that the soul is, in fact, the same as Kris, is difficult for many people to wrap their head around, so here is the same take written in perhaps a clearer if less flowery manner.
♥ THE BASICS
Kris = body (vessel) + SOUL
"Body" and "vessel" are synonyms. Every lightner has a "vessel" except for ghosts, who have to find theirs. That's how vessel was used in Undertale!
The body has, much like Pink's body, its own wants and needs - just like yours and mine. My will does not want to consume the whole pie at once, but I'm gonna do it anyway. Kris is only human.
_
One thing I don't understand. In the Weird Route, before chapter 4, soulless Kris had gone to Noelle to remove the Thorn Ring from her, tell her that it was all a prank, and warn her to not mention what happened. I feel like that doesn't really fit the idea that their actions are "predetermined by the plot" because they're a direct reaction to the deviation from the plot that is the Weird Route. I feel like at least in those moments Kris definitely had a will of their own opposed to the soul's, which I think is exemplified by Noelle noticing the difference between Kris' voice with and without the soul. Maybe I'm misunderstanding something, though.
Soulles Kris is by necessity plot-determined, because every event in the game is one of these: • player choice • programmed in (~predetermined) • RNG - never addressed so thematically irrelevant
Every character without the human SOUL is just another Spamton. They may think they're not, but the light world, too, is a magic circle, and the game really, really beats us over the head that "look! look! it's just like when they went to the dark world! it's screen after screen!"
Soulless Kris "doesn't have will" in the same sense that Noelle or Ralsei "don't have will" - they can act, but they can't decide. You can play the game 10000 times and they will do the same exact thing every time.
Notice that weird route here is the narrative transgression - and soulless Kris's actions resist it, they try to get back on track (and try to save Noelle, whom they care about). Noelle's "awakening" is signalled by her being able to break the game, force your choice menu and growing a red SOUL - powers like the player but slowly eclipsing yours - but also her becoming bolder and more assertive in regular life.
...soulless Kris's actions is also just what a person does when they fucked up really bad and try to salvage what they can. "None of this happened" - yeah right.