"Canonical, absolutely confirmed information about Gaster (meaning no speculation at all, regardless of how likely a theory is): Gaster was the former Royal Scientist. Gaster created the CORE and likely the True Lab. Gaster fell into the CORE and was shattered across time and space, effectively erased from existence, or perhaps more precisely, exists within a limbo, neither completely non-existent nor fully extant. Entering Gaster's name as the human's name, the vessel's name, or your name in either Undertale or Deltarune will reset the game. Gaster's Theme is, of course, the canonical theme of Gaster and can only be found in the Sound Test room, where the 4 songs Happy Town, Meat Factory, Trouble Dingle, and Gaster's Theme play, notably, you can switch between songs as you please, but after you select Gaster's Theme, that is locked in and you can no longer play other songs. Gaster has several followers who seem to also be in a similar state, and when you're in the same room as them, your phone no longer works. Gaster has an unknown relation to Sans, which is confirmed by the fact that Sans blatantly has an attack named a Gaster Blaster. Gaster's stats in the files were at one point 666,666 HP, 66,666 ATK, and 66,666 DEF, but it was removed, also, Gaster's name has 6 digits. And lastly, as well as (in my opinion) by far the most important information, is that Gaster wrote Entry Number Seventeen. We know this for several reasons. 1. Gaster is the former Royal Scientist. 2. Nothing implies Alphys can write in Wing Ding's and it would make no sense for her to do that when every entry found in the True Lab is written normally. 3. There is actually another Entry Number 17 unused in the game, which is quite clearly made by Alphys. 4. The room that you can find it in is LITERALLY called "room_gaster". Oh, and a bonus detail more than likely intended by Toby is that Alphys types the entry number using numbers, whereas the Wing Ding's Entry Number 17 is written using the word Seventeen.
So, with the confirmed info out of the way, let's get into the very likely theories, some of which are so likely in fact, that they border on being 100% certain.
First off, the Mystery Man is almost certainly Gaster. For starters, you remember how his stats explicitly included only the number 6? Well, the FUN value required to have a chance at getting the Mystery Man's room is 66. Not enough to convince you? That's fine, what about the fact that the teleport sound he makes is identical to the sound used for one of the Gaster Follower's? Of course, it's still fine to hold some skepticism, maybe these are all coincidences...oh wait, the teleport sound, when reversed, slowed down, and looped, sounds almost exactly like Gaster's Theme. Yeah, Mystery Man is Gaster.
Next up, Gaster is related to Deltarune. I am so confident about this that I am hard-pressed not to just include it with the facts about him. The voice you hear at the start of the game has a typer value of 666. A typer value is basically, to summarize it extremely, a specific set of data used for a specific character's dialogue. And 6 is, wouldn't you guess it, a number heavily linked with Gaster. Keep in mind that Entry Number Seventeen is only written in capital Wing-Dings, and the voice here speaks in all uppercase. Next up, the person speaking on Twitter using the Deltarune/Undertale account also talks in capital letters, not to mention that one of Gaster's infamous lines from Entry Number Seventeen is quoted. Oh, and I can't forget that, during the timeframe where those messages were sent, the name for the account was covered in blocky text, and the amount of digits were 6, the same as how many there are in Gaster's name. Not enough yet? Next up, have you ever heard of the Wayback Machine? It is a machine used to catalog websites and everything that goes on them. The Deltarune website has existed for a long time, and during its life span, there were only ever 2 images on the website, both very dark, but with Wing-Dings hidden in them, all of them being uppercase. The first one reads "THIS NEXT EXPERIMENT WILL BE VERY, VERY, INTERESTING." a direct quote from Entry Number Seventeen. And the next one reads "THREE HEROES APPEARED, TO BANISH THE ANGEL'S HEAVEN." a direct quote from Ralsei's story about the 3 heroes in Chapter 1. Still not enough? What about how, in Entry Number Seventeen, Gaster talks about darkness so much that it's like he's obsessed with it, with darkness being an extremely major theme in Deltarune?
Oh, and there's also a ton of evidence suggesting that Gaster is the Knight in Deltarune, which is an extension of the previous topic, so I'll go ahead and get into that, as well. First of all, you know how a mysterious man behind a tree gives you an egg? Well, this is something absurd, but when you type egg in all caps into a Wing-Dings translator, the resulting symbols are the same moves a knight can make in Chess. When you use your cell phone in a Dark World, a noise plays, the same noise heard in Entry Number Seventeen, which is described directly as "garbage noise". After you defeat Spamton NEO, if you return to his shop in the garbage dump, exactly 6 people appear, who knew Spamton. Apparently, Spamton was a salesman who couldn't get successful, and someone who called him was able to help him get big. But someday, Spamton's sales plummeted and his life was destroyed. The 6th one to the left says that he put Spamton's phone to his ear, as it was still on after Spamton had left his room, and when he listened to the audio, it was nothing but garbage noise. On top of ALL OF THAT. Several characters, including Spamton himself, have said lines very similar to what Gaster said in Entry Number Seventeen, primarily things relating to darkness.
Honestly, if you still think Gaster is unrelated to Deltarune after everything I've just said, I have no idea what'll convince you outside of direct confirmation in the game itself. And also, I haven't even gone over everything about Gaster. I've actually only dived into, at most, a bit over half of the iceberg. There is still much more that I could cover, but this comment is absurdly long already."
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