i have a theory in the video he uses undertale music alot and in the mystery ending it says stuff in wing dings who else speaks in wing dings a secret character you most likely know im talking about gaster but another thing that connects gaster is that in the files gaster is regarded as mystery man if it is gaster it would explain why they use the term mystery ending instead of secret and it refers to the character as him wich relates to how in undertale it never as been stated that his name was gaster but there was more like a cryptic code of secrets where you can peice them together to find out his name
Viewing post in mystery ending explained
true but gaster is also heavily associated with sans who is the finale boss in the genocide route wich means he is still considered connected and gaster is thought to be stuck in some sort of void wich could mean the basement could represent it another thing is that in one secret its known sans used to be a scientist and in other it also says gaster is one too,
of the dialog in the sans fight sans mention timelines which implies that he was a quantum scientist wich also can mean gaster was one too and it would make sense for a scientist to test things out in other planes of realitys so no harm comes to his reality right and in the game its implied that he "fell" in to his creation wich could mean a experiment with going to different timelines failed and he got stuck in the void.