What I think most people got wrong about UNDERTALE
I'm not sure if you can consider this a a theory, more of my interpretation of the story.
(Spoilers For undertale and deltarune chapter 1 i guess)
So...
Remember when everyone was arguing about whether Chara was evil or not, if they were responsible for the Genocide route (there might be some people who are still arguing), whether it was Chara or the player that's to blame? Well, I think both sides of the argument were looking at things the wrong way. I think the player is supposed to be Chara.
In order to explain I present "THE DIAGRAM".
If you can't read the chicken scratch I made in paint, don't worry I'll explain.
In Deltarune, there are three distinct entities related to player. The actual Player (red soul) the Vessel (grey kid) and Kris (the one that eats moss). At the very beginning of Deltarune, the player is directed to create a vessel, a body for themselves so they can interact with the game world. This vessel was meant to be your body, it would have been completely you. The vessel ends up discarded and the player is forced to take control of Kris. Through out chapter 1 everything is hunky dory, until the end of chapter one, where Kris rips the red soul (you) out of their body so that they can do what they please unimpeded, to show that Kris and the player are two separate entities. This is all surface level stuff most people understand this. Although more subtle I think this dynamic is what happens in undertale.
In undertale you name the fallen human, most people playing the game for the first time would use their name, or the name of one of their OCs, and again, everything is hunky dory, until you arrive in new home. This is where the player learns the story of the first fallen human and Asriel. Most players would get confused, if the fallen human died, how are they here now? This question is answered in the true pacifist ending, where Asriel laments that the human standing before them is not the one they knew. You find out that the person you have been playing as this whole time is actually named frisk, and the fallen human that you named died "a long time ago".
So Frisk and the first fallen human are different people, OK, simple enough, but then why do you play as frisk and not the first fallen human? that's the one you named shouldn't you play as them? I think you do play as the fallen human, or at least, what's left of them.
Here's where things get messy, and even I am not 100% about this. But I believe that, throughout the entirety of undertale, you are playing as the first human, and are possessing frisk through their determination. (from here on I'm gonna say Chara because its faster)
Chara was the character you were supposed to be in undertale, that's why you name them. Chara is the Link, Ness, Cloud, Red, etc of Undertale, they are your avatar. And every avatar needs a back story, such is the tale between Chara and Asriel. But in said backstory, Chara dies. Now this is a problem because how do you play a game where the player character is already dead? So the game can't start, until frisk, the eighth human, falls into the underground.
You, as Chara wake up when Frisk first falls down, it is their determination that wakes you up. You play the game controlling frisk up to the end of the game you watch the credits, and Flowey asks if you, not frisk, you, could get a better ending, or if was true pacifist, that you don't reset and let frisk and the other monsters have a happy ending. But you, being the curious scamp you are, reset, and go for a different ending.
This time you kill everyone, and after beating sans after who knows how many attempts, you come face to face with Chara, or whatever you named them. Chara talks about how confused they were that they woke up after their plan failed, about how your guidance led them down the violent path, how every time you felt good watching the numbers go up, that was them, and asking if you are ready to destroy this empty world and move on to the next. Most people, and understandably so, believe that is is Chara talking to the player, but what if this scene is not Chara talking to you, but You, as Chara talking to frisk. If Chara represents the player, why would they be talking to themselves, but if instead it was the body that they've been inhabiting this whole time, that would make a bit more sense.
Or, if you find the final cut scene hard to believe that their talking to anyone but you, it could be interpreted that reaching love 20 gave Chara the power to wrest themselves free from your control and becoming their own person, a la Kris ripping you out of their chest.
You are (or were) Chara, everything you did as the player, was Chara, everying that that Chara did, was you. Did Chara kill everyone or did the player, both of you did. Who pressed the reset button. Both of you pressed it. You are Chara and Chara represents you.
So in summary, In deltarune, you as the player are a different entity from Kris, you had a designated body but it was lost. In Undertale, I think Chara serves a role similar to the vessel, but died before the start of the game, and was only reawakened when frisk fell, and you used their body instead. You were playing as Chara, Chara controlled frisk, and Frisk's determination let all of this happen. I think this is what Toby was going for, but people didn't get it, so he made it extra clear in deltarune
Extra: Does this mean frisk is the narrator, idk. Does this make frisk evil, I don't think so. What's the red text, could be frisk or Chara. Did Frisk die when the fell, maybe, but probably not.
This theory kinda assumes or suggests that there is something akin to deltarune theorized DEVICE in undertale, I don't have an explanation for that, as far as the game is concerned, you play as a ghost kid, possessing another kid.