I want to preface this that this isn’t about the theory itself, rather it’s more about the Japanese involvement in making the theory more believable. There are plenty of other places where you can read about this theory in detail- a more recent place where I’ve seen it has been on the Deltarune subreddit. The post is located here!
Additionally, another preface: this will bring up spoilers for Undertale and Deltarune. If you are interested in the games, I’d suggest you put reading this on hold and come back to it at another point.
You still in? Good! Let’s begin.
The opening event of Deltarune is, without a doubt, creepy. You are being spoken to by a disembodied voice leading you through the creation of a ‘vessel’ for yourself. You create the form and ‘shape its mind’ in this sequence.
Of course, the follow-up to this is that once you’ve created this vessel, and done everything for it, it’s discarded haphazardly and you’re told your choices don’t matter in this world.
It’s a very strange scene, at the least. And very much out of character from the rest of the game, but it sticks with you. It’s not something that’s easily forgotten once you’ve seen it. And once you’ve seen the game’s ending, you find yourself looking back at this strange scene and wondering if they’re even related.
However, a theory that has come out that instead of one ‘person’ making you create this vessel and throwing it out, making all of your work for nothing (and is it really for nothing? Because the ‘creator’s name’ stays in the game), it is instead, two different characters. One guiding you through the creation of this vessel and another throwing it out at the end.
When you look at the game in English, it does seem that way? Or it could just be a stylistic change due to the change in the text styling. There is a lot in the way of oddities and trying to learn more of the story in just the presentation of text throughout the game. (Japanese screenshots will be captioned with the same line from the English version of the game.)
So let’s put the game in Japanese, and see what we get from it this time. Because that’s when things turn interesting, I feel.
To begin, this was translated from English to Japanese by 8-4. They’re a fantastic team and have done some great work- most of what we see, though, comes from what they’ve translated from Japanese to English.
This time, we get to explore an opposite situation.
When you start the game with the disembodied voice, you can see that the way of speech is not in a standard form of Japanese. Still using kanji, but replacing all hiragana with katakana instead.
The speaker in question is using ware, (我, われ), which is a polite, but somewhat old-fashioned personal pronoun. You may often see this presented in a case of an older speaker. Or someone who has connections to something older.
In this case, this one question from the speaker tells us that, on top of the usage of katakana, which I’ve seen used to show a somewhat ‘inhuman’ speaker, so to speak. Even though we’re in one of two games about monsters and humans, most of the monsters in both Deltarune and Undertale use hiragana when speaking, so that this is something foreign to even them, perhaps?
This will persist throughout the scene until the very end when the vessel that’s created is then thrown out. And that’s when we get the bigger change regarding this design choice than what we saw in English…
“No one can choose who they are in this world.” 
“Your name is”
The only way I can describe the first speaker’s means of speech is clipped- it’s not fancy. It’s not detailed. It’s straight to the point with little dallying (funny, considering how slow the text comes up on screen). Now we have a standard form of Japanese. You switch from kanji-katakana usage to standard kanji-hiragana usage. And to make it even more explicit that we’re dealing with a second character, the voice, in this case, uses omae (お前, おまえ) when referencing you. The original speaker used anata (貴方 (not as commonly written out in kanji, though), あなた, or アナタ in this case, as it was all in katakana).
Anata is the standard form in Japanese for you. It’s fairly common, it’s what you’ll probably be using yourself if you speak Japanese for most cases- and it will be what you will learn when taking Japanese lessons. Omae, on the other hand, is a bit more rough, and less-refined. It can even sometimes comes across as a bit rude, in some cases. Additionally, from my own personal experiences, I typically get an ‘older’ feeling (not like an old person sort of way) when I hear a character using ‘omae’ – someone who’s more mature, or sees themself as more mature.
This alone identifies, once again, the kind of speaker that we’re talking to- and definitely a different person than who was speaking to you, the player, at the start of this sequence.
So. What’s the point of this post? It’s not so much the theory, but it’s the really fascinating way of how Japanese reveals something that remains as vague when left in English. You could argue and debate endlessly on whether or not there are two different speakers in this case, because it could be a variety of reasons why you would go from capital case to sentence case in the text… but the Japanese version leaves no question about it from the pronoun usage and the style of speech used.
To me, the fun part of this is how different this is from the normal cases of what I’ve run into when translating from Japanese to English. Japanese’s vagueness and contextual reliance means that we have to spell out what’s intended from the sentence even more in English in some way or another. This may involve rewording, or it may involve added dialogue somewhere in the vicinity of the line.
This then becomes a rabbit hole, in a way, because of the forms of speech being such a big deal in Undertale in Japanese, as well. With this in mind, you can even perhaps start investigating who the voices speaking are in Deltarune’s opening using knowledge from both of the games- especially considering that 8-4 was involved in the Japanese release of Undertale, also.
In fact, seeing standard form Japanese like the second speaker is the rarity, and not the norm, in the game. In fact, the standard form you see in the ‘has now been discarded’ text is only seen once in the entire run of Undertale, and it’s when you are face to face with Chara at the end of the no mercy run of the game.
That’s very, very interesting, isn’t it?