0."Rules" in Deltarune differ from "rules" in Undertale - there is barely anything that suggests that Undertale and Deltarune has the same "laws".
1.Gaster DOES exist in Deltarune, but is fundamentally (i.e. in the context of storytelling) a completely new character in Deltarune (just like Toriel in Deltarune is fundamentally different from Toriel in Undertale).
2.The soul represents the player's connection to Deltarune.
3.The Eggs are just a completionist's delight. So are the Shadow Crystals.
4.Mike is not a character at all, but is Tenna's way of ordering everyone backstage. For instance, "MIKE, play the VHS!" is roughly equivalent to "HEY you SLACKING guys backstage, MOVE your a*ses and play the fricking VHS RIGHT NOW."
5.The Roaring Knight is NOT Asriel/Dess/Carol/Rudy/Kris&Noelle/an amalgamate/any other theory that suggests that The Roaring knight is a character that have already appeared in Undertale or Deltarune Chapters 1 to 5, but an ENTIRELY NEW CHARACTER, the specific identity of whom has never showed up at all. (Side Note: aaaaand Toby Fox might never reveal who the Roaring Knight is lol)
6.1225 means nothing except December 25th (Christmas) and a possible candidate to one of the codes to the lock of the shelter.
7.The Prophecy is a lie. It either scares people who saw it into following it against their own will, OR it can be changed (the Old Man is always right, I suppose!)
8.The one "ending" is the game ending (or, to the computer nerds, "taskkill /f /im DELTARUNE.exe" lmao)
WARNING: These are all only theories, and game theories, which are never subject to normal logic, are fragile. They may have their own faults, or even are completely wrong. But theory 4 and 5 suggests that maybe, just maybe, we are mistakenly making characters like Gaster(Deltarune equivalent), Mike and the Roaring Knight too mysterious than necessary.


