I don't normally write down theories so this might not make sense, but i kinda like this idea and wanted to share somewhere.
Also sorry for the horrendous grammar mistakes since english is not my first language ._.
This is based on top of a already existing theory, the bread crumbs theory.
Basically, What if the faith of Gaster in Undertale is the same as the one to Noelle and Kris on the lake In the weird route?
So, some of the goner npcs on Undertale tell how Gaster was the original royal scientist and that, in one day, he "fell into his creation" and dissapeared without a trace, but what if him falling in the core was considered an "unintentional" place to go or to die by the game's code and made him have the same fate as us in the lake? Considering the bread and crumbs theory, Kris and Noelle broke the bounderies of the game by crossing the lake, suposetly just like Gaster did.
Most of the goners sprite are depicted as monocrome and warped versions of already existing characters in Undertale and Deltarune, something that seems to reference earthbound's bread crumbs bug black and white map colors and broken textures. This could aline with the story of Gaster/mystery man dissapering from their reality, maybe glitching into another broken version of their timeline, explaining why the Deltarune chapter 7 is also called "side b", being an alternative version of the normal route ending. Maybe we could even see an goner version of Kris and Noelle, what if the vessel we create at the start of the game is the goner form of Kris that appears in the final chapter?
Also, Noelle thought the game (and the spamton sweepstakes) is said to have interest in videogame glitches, like when she talks about Dragon Blazers were Noelle says how she likes to use buged doors to skip important gameplay points (just like how we skip chapter 6), probably comming from her interest of doing something different from the expected path, in a way to break free from her monotone life in the real world.
Now, this part is more speculative than the rest, in a way this could also be tied to the roaring knight. The knight is a really weird figure, they can change their shape, become static when hurt and even open their rib cage with is not something any normal lighter has done yet. They seem to not be a darkner as well since they can walk on the light world (also somehow they can carry Undyne and Asgore around hometown).
But something i find more interesting is that the knight also falls in the theme of a monochorme appearance that the goners and the Mystery man have.
In the battle agaist the knight, their final attack is very similar to the hyper goner attack that asriel uses in the pacifist route in Undertale, along side that, they also have the same holes in the hands as the sprite that is suposetly used for Gaster (it's also weird why they have that detail in the first place). This connections seem to imply that the knight could also be a type of goner.
Most if not all characters in the game don't know the knight's identity maybe not just because they hide it, but because they never existed in the first place, just like how Gaster was forgoten, but unlike him, the knight is capable of showing themselfs in the real world after the case (or the others goners choose not to do so), that way they build a new identity on top of the one that was deleted by the world.
