This is a reddit post I’ve written myself, and I’ll just be copy-pasting it (since I’m lazy and don’t wanna retype a huge essay. like hell no.) Apologies if some mistakes/contradictions are presented, but have a read. It is a long post, so grab something to eat or whatever.
“I recommend reading the post in full detail, but TLDR:
Deltarune is basically Gaster VS Chara. Takes place on another world after a post-geno run where you never return to the game (Undertale), therefore letting Chara destroy other worlds.
Deltarune (Chapter 1) is a world where Asriel isn't dead, Toriel and Asgore are together (i think?), Alphys and Undyne never knew each other etc.
Gaster introduces you, but Chara cuts the link between you (the player) and him (Gaster).
Sans will play a major role sometime, most likely in the next chapter (obviously).
The body we've created is our vessel to meet Gaster in person, as we cannot go to the void in any body.
Frisk and Suzy are like constantly changing variables in every world. One world could have them with a different name, age, backstory (thus, we have Kris and Susie).
I have multiple theories about Chapter 2, and I can't summarize it, so have a read on em.
I'll try and make a clear point firstly:
Deltarune is basically Gaster VS Chara. It also takes place on a post-Genocide run where YOU NEVER RETURN BACK TO THE GAME.
When you open the game, you are obviously greeted by Gaster. This is evident by the fact that he speaks in uppercase letters, and was pretty obvious since he does the same on the Twitter posts. His wording is also very much like him:
EXCELLENT. TRULY EXCELLENT.
So obviously, we can tell this is Gaster speaking as well as his theme playing in the background.
Now, I know you all are probably like "Yeah, it's obvious.". The next thing however, people might have not noticed this.
Read the text from the intro of the game once again:
No one can choose who they are in this world.
Notice how the text suddenly becomes properly formatted. The text also has no sound effect (tbh, neither did Gaster), and the most noticable thing is how the background changed to black, similar to how the Genocide run ended. The way how the speaker talks is also very much like Chara.
What happened in that moment, was that our connection with Gaster has been destroyed. Chara had interfered and took control of the situation. What ended up as a simple survey program for Gaster, ended up being another tool for Chara to use to make us help them in destroying more worlds.
So now whats next in Chapter 2? Chara is on the loose trying to destroy more worlds, and Gaster is trying to stop and "fix" everything.
For this, I have these theories/speculations.
Chapter 2 picks right where Chapter 1 ended. You might be controlling someone else as Chara possesses Kris, or you're controlling Kris alone (most likely) however, your control is split between you and Chara. You're continuing on with Sans' request to visit him. He ends up getting you to the secret lab and gets you to the real deal about Gaster. And things start to go from there.
Deltarune could be a series of events. It doesn't have to be in the same world. From what we could know, Kris/Chara could've already went into killing as the Chapter ended. Chapter 2 could start on a new world. New twitter posts would share the same "connection" thing (assuming chapters are the game's structure, and would be released one at a time.)
Note: This theory can kind of fit into any of the previous theories.
Firstly, is Gaster even a "Darkner"? To me, he isn't. Gaster and co. are beings beyond the norms. Each time Chara destroys a world, a few of them get sent to this "void" where Gaster belongs. They become shattered in time and space, and thus why they're exactly like their colored counterparts in Undertale. They're versions who had been involved in Chara's destruction.
Sans is strongly involved in this situation, acting as a "middle-man" in this conflict. It is as if he's a servant to Gaster, since Gaster himself cannot do anything in reality.
I feel like this "Lightner" and "Darkner" story/concept only exists for this specific world. It feels really... out of place for Undertale. It is basically that world's version of the Human vs Monster war. Probably more evidence that this is indeed taking place in another world.
As for Susie and Ralsei, I dunno. I feel like if Theory 2 is correct, it just cuts all that character development from the both of them. Doesn't really make sense for me, so they may end up becoming "followers" if they become "involved" or they somehow get preserved.
The Suzy and Susie discussion:
Many theorists have been pondering with this. Are Suzy and Susie the same person? I might have a crazy theory for this.
What if Frisk, Kris and Suzy, Susie are linked together?
I mean, look at this. It is said by Clam Girl that Frisk and Suzy are about the same age. Kris and Susie are ALSO the same-ish age considering they both go to school at the same class.
What I'm saying is, Frisk and Kris are essentially the same (and so are Suzy and Susie). They almost have the same traits (Frisk is a lot more player-defined). It is only these characters that get changed in every world. Kris is Frisk from another world, and Susie is Suzy from another world.
What if, THEY are the two that are the "WHAT DO YOU TWO THINK?" in Entry Number 17.
So what about the body we created in the intro? I theorize that body, which is called a vessel by Gaster, will be, as he said, a vessel for us to enter his realm, The Void (lets call it that). It is grey-ed out like all other NPCs, and himself. This body could probably contain the same properties that they all do. We will be using the vessel to enter his realm to have a chat with him.
I think this will all lead into a final True-Pacifist ending. Chara and Asriel are both saved. Gaster and Frisk are alive and back. Everyone will be happy. This may be the final episode for the Undertale franchise (who knows?)
