Undertale is a game whose story has several particularly 'meta' aspects, i.e. narrative events and details that either reference or outright acknowledge the fact that Undertale is a video game.
This document attempts to establish and define several terms and scenarios, and use these definitions to analyze Undertale 's diegesis, namely in regards to the player's relevance to the story.
Before proceeding, I'd like to acknowledge Andrew Cunningham's video regarding Undertale 's diegesis:
What was so good about UNDERTALE, anyway? (A mildy deranged retrospective)
The following document regards many concepts that this video introduces (and explains in much finer, more comprehensible detail), so I'd highly recommend watching it before proceeding.
The game world asserts that the player and their world is non-existent.
The player may be meant to embody a particular character, perhaps even crafted by themself in their own image, but the player themself, as a human playing a game, is not real.
The game world acknowledges that the player and their world is existent.
The player is expected not to embody any character whatsoever, outside of themselves. The game world acknowledges the player as a human being in a different or higher plane of existence, even if the characters themselves do not.
The game world asserts that the game world and the events therein are real.
The player is expected to suspend their disbelief over the existence of said world and events.
The game world acknowledges itself as a video-game, thereby denying its own existence as a real world.
The player is not expected to suspend their disbelief over the game world or its events.
This is, in effect, reality; the true state of events.
Video games are not real; you, reading this, are. Any game asserting this scenario would thereby accept its existence as a video game.
The standard state of the suspension of disbelief.
The player (or audience of most pieces of fiction) is expected to temporarily forget their own reality, and suspend their disbelief over the unreality of the fictional world and its events.
In the particular case of video games, the player may be meant to embody certain characters (Link in The Legend of Zelda ; custom characters in, say, Terraria ; etc.). This, however, does not mean the game truly acknowledges the player or their world as existent.
This is a unique state of events wherein the game world acknowledges the player and their world as existent, whilst simultaneously upholding the reality of the game world and its events.
This then implies that the player, although expected to maintain their existence, is also expected to believe that they are not simply playing a game.
For example, they instead may be expected to believe they are using a program on their device to monitor and influence events in another parallel reality or far-away place.
This would imply that the player is expected to forget their own existence, while simultaneously not accepting the existence of the game world.
I don’t know what this entails, exactly. Moving on.
Firstly, let's establish Undertale 's position on its own world: it's fairly clear that Undertale believes its own world to be real. There’s no indication that it believes itself to be unreal, and more than enough reason to think that it believes itself to be a real world.
Now, let’s discuss Undertale 's position on the player and their world (i.e. our world); this is where it gets a little unclear.
Here are some key points to keep in mind:
The first thing to note about these is that Frisk is evidently not simply 'us'. The True Pacifist route makes this fairly clear: Frisk is their own person, who we simply control.
From there, ignoring points #4 and #5, it seems reasonable to think that we are therefore meant to embody Chara:
The player 'is' Chara, who then controls Frisk. Note that this is effectively Real World Non-diegesis, albeit with a protagonist that is analogous to an actual player.
This is a slightly bizarre arrangement, sure, but it very naturally explains Chara's name appearing throughout the game, and why Flowey outright calls us 'Chara'.
Point #4 complicates things, however. After all, Chara speaks of themself in the first person, while addressing us in the second person, even going so far as to outright state "You and I are not the same, are we?".
Not only that, but they're specifically addressing the player:
"There is a reason you continue to recreate this world."
"There is a reason you continue to destroy it."
"You are wracked with a perverted sentimentality."
(?)
Note that this dialogue is exclusive to doing a No Mercy run twice.
Given that Frisk doesn't seem to remember things across true resets and/or destroying the world (they don't know to turn around to shake Sans' hand before he says so, for example), it's safe to say this line can only be referring to the player.
This then implies that the player is diegetic to Undertale. Either we diegetically control Frisk, and Chara's along for the ride, or we diegetically control Chara, who then controls Frisk.
Alright, now that we've taken inventory of (most of) the relevant information, let's consider our two major questions:
From here, there's four possibilites:
Now, let's dissect each of these options, and consider what they entail.
This possibility is a pretty natural way to have the player control Frisk without 'being' them or Chara.
However, this scenario assumes that:
This scenario implies that the player controls Chara, who then controls Frisk.
This adequately addresses Frisk and Chara as separate entities from the player, Flowey's dialogue, and Chara's name appearing on the SAVE File and in battle.
However, this scenario is bizarrely complicated; it introduces a lot of narrative complexity for seemingly no reason, aside to account for various independant details.
That is to say, this feels less like a cohesive narrative idea and more like a band-aid covering up a plot contrivance.
This makes this scenario less desirable, but not impossible, especially given other Undertale plot contrivances (see: essence lore).
This scenario assumes that the player is non-diegetic to the world of Undertale, and is simply meant to 'be' Frisk.
This is by far the simplest possibility, and is arguably what the player is supposed to assume for the majority of the game.
However, in my opinion, this scenario is the most flawed of the bunch, and by far.
In addition to the concessions of a diegetic player controlling Frisk, this scenario also assumes that the True Pacifist route's insistence that Frisk is their own person is a one-time fourth wall break, and that Chara's No Mercy monologue is (somehow) referring to Frisk as opposed to the player.
This scenario implies that the player is meant to 'be' Chara.
This very naturally explains their name's presence on the SAVE File and in battle, in addition to being corroborated by Flowey's post-pacifist speech, and accomodating for Frisk's individuality.
However, it largely ignores Chara's No Mercy monologue.
For this scenario to work, you have to either assume that it's a one-time fourth wall break, or that Chara is speaking to Frisk (again, somehow), and that the player has simply switched perspectives for some reason.