To not spam the already huge Demiurge thread, I will create another thread dedicated only to the Friend Knight theory.
I had this theory that the Roaring Knight is Friend (who is also probably Mike), and because he is somehow involved with Gaster it makes him a traitor to his own kind.
Recently, I stumbled upon this interesting and first ever video about Friend Knight on youtube, and oh boy the author pointed out things that even I have missed while playing the game.
First off, Friend is certainly behind the shadow crystals, as he could've easily placed the shadow crystal inside the machine and tell Spamton about it. And Roaring Knight actually seems to generate them out of thin air, as you get one out of nowhere after beating it in C3, and in C1 Jevil has it and it the Chapter it's implied that he was interacting with the Knight. A stretch? Maybe
Second, ERAM doesn't actually exist. The Mantle minigame is orchestrated by Friend, as the dark rooms in thefirst board show us: eyes of Friend(1), smile of Friend(2), and the middle room has ERAM's dark text stating "Having fun?"(3). It's something that I completely missed out when playing the game.
However, I actually found even more subtle details. C3 starts off with showing us that Friend is an amalgamete of sorts, and in UNDERTALE amalgametes hid themselves in ordinary objects and furniture. It would explain why there is SMILE item in the vending machine outside Mantle room that disappears once you complete the minigame, Friend was hiding in there. And if you damage the Shadow Mantle in the minigame, the screen glitches as if you're fighting the game itself. As if Friend was possessing the game itself.
Moreover, in the last phase of the Shadow Mantle boss, the textures change to crying eyes of the Roaring. This indicates to me that Friend not only laughs, but cries. And it would make sense if he was the Roaring Knight, because the Knight both cries and laughs from pain, like Gaster.
"But muh muh, the Knight was late"! "the chapter is called Late Knight u stupid!", uhhh aren't you forgetting something important? Knight and other possible intruders could've easily enter from the window, Kris left it open in C2.
But you could also say that if this theory is true, then writing of DELTARUNE is shit. I agree. And so I hope for these statements to be false.