I feel like if you wanted to adapt Deltarune into like a comic or a novel or whatever, you'd need to do something very different with the Soul, because the game is riffing on "you, the player, control a character", and that's just not a thing in non-interactive mediums. That sequence in Chapter 4 would be particularly troublesome ...
Like. "Kris rips their soul out"/"Kris is possessed by an Entity" is surprising on its own, but if you really want to preserve the original twist, you need to go for something centered on "who the protagonist is" which is specific to that medium and is so fundamental as to be invisible, just like "player controls character" is in a video game.
The first idea that I'm halfway satisfied with is a first-person narration/internal monologue, carefully phrased so that "I" seems to refer to Kris, right up until Chapter 1's ending turns that on its head. But that might not work as well in a show or whatever where "narration" isn't standard.