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