It could be that he's narrating.
(Assuming it is indeed Gaster....)
If Gaster was in control, why say these things?
He makes a big deal of being connected when Deltarune is first run, so clearly he values some form of interface between himself & the SOUL.
Like, when copying to another file, he states "IT WILL BE SUBSUMED.".
That's a warning/cautioning; The phrasing implies confident knowledge of what will happen.
Or trying to copy an empty file. "IT IS BARREN AND CANNOT BE COPIED.".
If the copying action fails because the file is barren, there wouldn't be much reason to say that if he was experimenting alone, since it's phrased as though as it's already known info. (Not to mention it seems like fictional scientists, when acting on their own, often pursue new info, rather than rigorously going through all permutations as the scientific method might encourage. Gaster might actually be so thorough, but until we know otherwise, if such a trend is true, it could be presumed for him, too.)
Gaster is an experimenter, a scientist. He wouldn't need to warn himself if it's himself doing the experiments.
There's also the overwriting when all files are the same dialogue, saying "WHAT AN INTERESTING BEHAVIOUR.".
Not to mention the cursor being the SOUL. What things are involved in each experiment in that file select is dictated by the choices of the cursor/SOUL, which isn't controlled by Gaster. (AFAIK.)
Why he wants/needs our help for that, IDK.
It is curious how some of it is phrased. Such as "BUT THERE WAS NOTHING LEFT TO COPY.". Feels kind of past tense. Meh, IDK.
Other curious stuff is for after copying -"THE DIVISION IS COMPLETE."- & if all 3 files are made the same, it says "PREPARATIONS ARE COMPLETE".
Is Gaster announcing this as though thinking it's our preparations?
Does he seek to have 3 identical files for some reason?
Copying a Save File divides it? Is it not a timeline? Is it some kind of simulation computational power?
& why is an EMPTY File barren?
I know from personal experience that you can beat Deltarune -At least as far as the end of Chapter 2- without Saving. (Although, IIRC, Jevil & Spamton are agony. & also those danged teacups.)
If you do that, your file is EMPTY.
Barren of what then? Determination exertion?
I suppose since it's a post-chapter-starting pre-completion menu, you can't be there having beaten with an unsaved-to, EMPTY file. There would have to be progress or you'd be at the regular menu, right? Chapter Select?
Is the timeline like the Vessel, & artificially generated? Is that why the voice knows of Kris, Susie & Noelle? (If you name the Vessel the same as them, it says you're about to meet someone interesting, IIRC.) But then why no such reaction for Ralsei?
Is Ralsei so tragically forgettable even to such a dedicated researcher? Is Gaster only concerning himself with informing about Lightners? Are Darkners somehow unknown to him?
Is the SOUL maybe somehow his means of getting info about Darkners/Dark Worlds? Or does he want Ralsei to be a surprise to us?
Hopefully no one minds my yapping.