Depersonalization and Deltarune
So, just to start out, I'm not setting out to diagnose any characters, just to provide my own interpretation based on my own experiences and understanding. I have a lot of experience with dissociation, specifically depersonalization and derealization (DPDR). I won't go into depth about dissociation because I think it's pretty commonly understood at this point, but depersonalization is kind of like a product of chronic dissociation.
Dissociation is a tool. It's the brain's way of dealing with input that is overwhelming. It turns down the volume on the senses, emotions, and even perception of pain. It's a way of getting through the unbearable. It's typically a response to chronic stress, or trauma - if someone is going through a difficult situation, it can be a way of the brain easing the burden. But the difficult thing is that the brain can't be selective. When you get rid of the bad, you also tune out the good. We can't really decide when it comes and goes - in fact, trying to control it can often make things worse. And we can certainly see at this point where the trauma may have come from for Kris - a messy divorce, friends disappearing, bullying at school, a mother with problems with alcohol.
And here's where I think it closely ties into the themes of Deltarune. There is a weird phenomenon in DPDR where the body seems to act on its own. People describe it as feeling like a robot, or like there's someone else taking control of their actions. There's a quote in the book Overcoming Depersonalization Disorder by Fugen Neziroglu and Katharine Donnelly that I think of a lot when it comes to Deltarune (bold my own):
"When I'm around a group of people, it's like my brain checks out. I'm no longer there but somewhere slightly outside myself. I'm able to watch my reactions and to understand the feelings that should be there, but I suddenly feel that I don't have control over what I do. It's as if I've made a prior arrangement with my body about what it will do, as if my reactions are socially acceptable but aren't immediately coming from me. Feeling like you're not the one in control makes you feel that you're totally out of control, even if everything looks calm from the exterior."
So, thinking of the soul's relationship to Kris in Deltarune. It allows them to do things they ordinarily could not. They can seal dark fountains, they can shine their power on other party members to enable them to act. They can take damage. The soul can also do things that Kris may not want to do (or may not want it to do), such as in the case of the weird route. It's kind of like a Persona (the Jungian psychology concept and in the JRPG) - it takes the slings and arrows of life instead of you.
But dissociation has its costs. It runs your body at high stress levels. Your body is focused on survival. Your hunger and thirst levels go down. Your pain tolerance goes way up, and when you come back to normal whatever wear-and-tear you took without noticing will still remain. That's why whenever I see Kris absent the SOUL, it makes me think of what it can be like to come out of a dissociative state and all of your needs come flooding back. Your emotions return, you're likely in pain, you may be starving and will take whatever food you can get. And you still have a memory of all that you experienced during your dissociative episode - embarrassments, things said that you wish you could regret, it all comes flooding back. And then you might kick a trash can a dozen times.
Anyway, to reiterate I don't think that Deltarune is necessarily inspired by Toby one day reading the DSM-IV, but when it comes to human experiences it's helpful to remember that dissociation and depersonalization are a part of it - everyone has likely dissociated, and many people have likely had a time with depersonalization at least once in their lifetime. And while it can be interesting to theorize about who may have created the SOUL or what its grander purpose in the plot may be, despite its occult or science fictiony trappings, it touches on something that is very human, and that I've found very resonant with my own experiences, and has helped me to understand them.