Kris and the soul are one and the same, and that's why Kris hates it so much
a long-winded diatribe
This is my hill to die on.
Kris is not possessed by an extradimensional heart-shaped ghost, Kris is a lost teenager full of self-hatred, inner conflict and self-control issues.
The body and soul are two parts of Kris. The soul does not take agency away, it is agency. The soul is not Kris's possessor - it is Kris. The soul is not the player - Kris is the player because they have the soul. And that's exactly what Kris hates.
THE PHRASING
First, we choose everything. Our body, our mind, our likes, our name. And then we hear:
This is the statement of theme. Front and center. What this game is about, is choice to be who you want, or lack thereof.
to paraphrase, "You (player) are Kris. You like what you like - whether you like it or not. You want what you want - whether you want it or not. You have many needs that you don't need. You didn't choose your name or species, and most definitely not your fate. You didn't choose yourself, but that's who you are."
Kris is not a blank slate, but they are still you. If you don't like being Kris - how fortunate! You have that much more in common!
Narration repeatedly conflates you and Kris, speaks of your body parts acting on their own. The soul, too, is phrased both as "Kris's SOUL" and "Your SOUL".
Many claim the narration is unreliable, but ignoring verbatim text is an easy way to justify anything. Narration has never provably lied to us thus far.
Ralsei, too, drops these gems right at the beginning of the game:
And this is what we see in a book:
What if they don't lie? Would the game still have sense and thematic weight?
WHEN AND WHO DOES KRIS DISOBEY?
There are few things we actually choose, and most of those we end up doing more or less literally. Most player options are what Kris can conceivably think of and say.
Many options will be mumbled, rephrased or resisted - which is something real people also do. Have you never struggled to say something, even if you mean to? Have you never started saying a sentence, only to realize it's kind of fucked up and corrected yourself?
Have you noticed other characters do it as well? Sometimes they even have a different font for it. Noelle does it a lot.
The more emotional conflict Kris has over an action, the more likely it will fail. Of course Kris would be tempted to look at their brother's love letters, look in his room in Queen's palace, or read a book on humans. Of course they'd also have difficulty doing so.
Why couldn't we read the letters before? Why only now? Because that's not a thing Kris would do. Not then. They didn't feel like it.
VISUAL METAPHOR
In any other piece of media - without the baggage of Undertale's meta commentary, if you saw a character rip their heart out and toss it in a cage, what does that mean?
Would it help the case if they're a traumatized, alienated, mentally ill teenager with a broken family?
They feel like shit, so they lock their heart away and indulge in some pie. When the most powerful adult in town makes them betray their friends, it will at least ease the pain to toss it in a box and get shitfaced on chocolate syrup. If you must obey, it's easier if you can't refuse, if your volition isn't home.
When Susie asks to walk her home, "not now" really sounds more like "we don't wanna end the chapter quite yet" - much like previous chapters. But it still feels bad to say "no", so Kris immediately regrets this choice, gets rid of their agency and complies.
What does it mean to beat your heart up? Does it matter, that Kris does it while Noelle - their old best friend and crush, flirts with Susie - their new best friend and crush? When Noelle calls them weird, says their cherished moments were forced? When they're busy betraying Susie at the same time?
What about after doing something really, really wrong to your friend?
All of this begs the question - why would Kris do any of these horrible things? Why would the player do any of these horrible things? I don't know. I didn't, after all. I walked Susie home and never played snowgrave. But there are people who do, and Kris can be like them, too.
***
Within Undertale's baggage of meta commentary, Kris is the Chara parallel. We know they're capable of (becoming) far, far worse with no player involvement.
None of these are conventional ways for video game protagonists to behave, but Deltarune is hardly conventional. It's also not the first, Disco Elysium's protagonist has at least 27 voices in his head, and the player is one of them.
ASCETISM
Kris will not love themself of their own volition. Why would Kris think they deserve to be happy?
They don't. That's why Kris refuses to play piano. "Concert for you" - a concert for Kris, the only kind of music that they deserve. But they will play on two occasions: 1. The soul is gone - and with it, emotional turmoil 2. They have no other choice - the best excuse under the sun God forbid they enjoy themself.
Of course their room is plain, it's a visual reminder that Asriel is so much better than us. In trying to emulate their angel of a brother, Kris only ends up a horned devil child. Toriel's room is locked to keep the sweets from us. They can't have nice things because they're a sinner - something the town priest does not believe in, but Toriel does, Asriel did, and so Kris probably does too - whether they want to or not.
It takes Susie to get us the first wall star and clean up the bloodstain. Ralsei gives us our fully decked out room.
Susie and Ralsei have similar struggles with their sense of self-worth. Susie's room is even more barren, she hasn't known much love in her life and can only express her needs by force or threat. Ralsei's is just a cell - he has no compassion for himself and must learn that he deserves comfort at all.
Ralsei going splat on the floor is funny and all, but he does it for pretty sad reasons.
Susie and Ralsei don't lock all their niceties away so an extradimensional meta-entity can't spy on them. They don't have niceties to lock away. Neither does Kris.
SPAMTON
Spamton is no counterexample.
Spamton's one real signifier of unfreedom is the strings, which could mean many things. He thought power would make him free, but power comes with strings attached, said almost verbatim right as the fight begins:
Spamton is a puppet. There's no mention of a puppeteer. He didn't choose to be this way, and he does not like it. He thinks, if he becomes big enough of a shot, somehow his Spamtonhood will pass too - but he can't choose that.
No split personalities, no foreign entities talking for him. His new body and even its conspicuous pop-out heart-on-a-string both have the same face, his face, as if to make it more obvious.
He is a chained prisoner inside himself, a cage for his own soul. Spamton can see his strings, but everyone has their own. Agency is a very scarce resource in Deltarune.
So, where now does Spamton want to obtain his agency?
Oh, right. The game does, after all, tell us numerous times that the SOUL is a source of agency.
Something we do actually see, is Spamton addressing the heart as Kris, and still implying they are not free:
Silly strings, garbage can and heart on a chain being obviously himself projecting onto Kris.
PINK
Pink is a lot less ambiguous.
She is composed of a soul and body, but they are the same person most of the time. They split when the soul's fears and body's desires conflict.
Pink has visible strings like Spamton.
There is not even room this time for a spooky off-screen puppet master, no mysterious phonecalls, no ties to Gaster or the Knight. The only puppeteer here is the ghost, who is also Pink herself.
She's repressing her feelings, that's all. And how does she receive closure? Learning to accept her own duality - acknowledge both her body and soul's needs.
Much like Kris, Pink also wants to belong within a group (monsters/flowers) that is not her own kind (human/ghost).
***
In neither of the above cases is the soul a harsh master usurping control. It's always a prisoner. The body is the cage. But the body is also you.
CONCLUSION
The soul is Kris, and it is Kris's agency. Kris has the power to choose, that's why they are the player character.
This reading is the most consistent with the text, relies on the fewest assumptions, and it's the one that underlines its central themes and emotional arcs, rather than get lost in theorizing about extradimensional possession or guilt-tripping the player.
Making the player into a foreign entity, extracts all the most interesting parts of Kris - the internal conflict, self-hatred, emotional back-and-forth - and offloads it onto conflict with an external force.
The notion of agency, choice and freedom in Deltarune is a lot more nuanced than "freedom=choice=agency is when noone tells you what to do and it's good". We are repeatedly shown characters, who are prisoners of their own mind, who become less free by pursuing freedom, who find freedom in constraints, or who refuse to be free, whether it's good for them or not. Agency is not always what you want, and not always what you think.
You may not like seeing it, but still, it's what they call "you".
