Hello, it's me, Dio.
And I'm here with a theory on how to crack the code of the man who speaks with hands only.
Also known as Gaster.
I have so far been able to crack multiple parts of his text.
This theory will be based around what he says once you decode it.
What is Gaster you ask?
He is in fact, the judgement maker of Undertale.
Well, the second bringer of judgement I should say. Sans was the first.
There is a good chance that Gaster IS an anti-hero version of an angel!
Now let me explain a bit before you all start commenting "This theory is false Dio".
One of the things he says before the battle is:
"Who am I"?
"For now, I am nobody".
"Only someone who wants to have fun".
This phrase means that Gaster has bad intentions for you and if it's like you, then you must have played the genocide route in order to have "Betrayed their trust" as he says. Or to have "Betrayed their hearts" as he says.
It's possible that Gaster is the result of the fallen child first going through life as a pacifist, which earned all of the monsters trust, then went through life again as a genocidal maniac, betraying them as Gaster said.
I will finish this first part by giving a list of what all Gaster says in order from first to last. (I will also explain how eac piece assists this theory)
"Satisfied"? (Asked to the genocidal child)
"Bored"? (Also asked to the genocidal child)
"You did this". (This is the judgement part)
"It cannot be undone". (Again, the judgement part)
"... this was when you sold your soul to Chara". (And this just hammers it home)
"For a timeline to destroy". (What did Chara try to do?)
"That didn't happen". (Interesting)
"Chara is gone". (?????)
"And I'm here". (You killed Chara?)
"Who am I"? (he wouldn't reveal himself right away)
"It depends". (Depends on what?)
"But for now, I am nothing". (Again, keeping up the disguise)
"I'm only here to have fun". (Fun?)
"Just... like... you...". (Oh... shit...)
(Battle Time!) (I put my reactions in parenthesis)
("Lets do that again" happens if you die)
"Expect a challenge"? (No, I expected Sans to come out of your costume)
"Prepare for disappointment". (You're easy?)
"Because I want this to last". (Oh, it will last buddy)
"Normally you would die on sight". (Yep, that face is pretty scary)
"But you were special". (I just put a blindfold on)
"Usually you kill and move on". (Yep, follow the system)
"But that didn't happen". (I broke the system?)
"And you do not deserve a quick death". (Aw, did I hurt your feelings?)
"Disgusting, how you toyed with them". (Them? All I see is me and you buddy0
"Their emotions". (Their? Is your mother joining us?)
"Pretending to care". (Yeah, I don't really care about your mom)
"Betraying at highest hope". (That's what she said)
"Lying at every chance". (That's also what she said)
"And all I could do was watch". (Is that your primary skill?)
"As you acted these terrible fantasies". (Oh yes, killing Toriel and Papyrus and Muffet and Mettaton and Asgore... except those weren't fantasies... I MADE THEM INTO REALITY!)
"...". (Scared much?)
"Until I was freed". (Who was that dumb?)
"You want to know how"? (Indeed)
"Just know a little white dot helped me". (I knew it, you're an inanimate object whisperer)
"Strange..." (Not as strange as your language buddy, I can promise you that)
"You should be dead...". (But I'm here. How do you like your own medicine?)
"Survive any longer and I'll have to use my special attack"! (Oh I'm so scared. It's sarcasm asshole)
"I'll give you a chance to die". (No thanks bud)
"YOU ASKED FOR IT"! (Indeed I did)
"Here we go". (Ripping off Sans much?)
(Gaster's Special Attack)
"Time to die". (Nope, it's not midnight yet)
(After you beat him)
"So you are a threat afterall". (Thanks for noticing)
"But who said you have to die"? (You did dipshit)
"You will be stuck here forever". (Or will I?)
"Goodbye". (Bye asshole)
Alright, here's part 2.
After a careful analysis on Gaster's text, I still think he may be a sort of angel of judgement for Undertale.
Sans knows him, but tells you that there's not much point in talking to him. I wonder why...
Maybe since Sans seems to know a surprising amount of information about the human world, he may know just WHO Gaster is and may know what Gaster is capable of. Let's think back to my theory I made on who Sans might be.
if Ness from Earthbound is Sans and Lucas from Mother 3 is Papyrus, there's only one main character left. EarthBound is the 2nd game and Mother 3 is the 3rd. What about the first game you ask?
The main character of the first game was Ninten. (obviosly a Nintendo fanservice to themselves) His PSI powers are STRONGER than Ness and Lucas's by a consideraby amount. While Ness was lost in time and Lucas ended up as the hero that ended the world, Ninten mysteriously disappeared with a trace.
Gaster is LEAGUES stronger than Papyrus and is quite a bit stronger than Sans as shown during the fight with him when he almost manages to overwhelm Chara with his attacks. Something that Papyrus and even Sans couldn't do.
Although Gaster doesn't kill Chara, Chara doesn't kill him. Making him one of the only monsters that survive their encounter with Chara. Which makes Gaster a tougher opponent then both Sans and Papyrus. Appliying that along with the 3 main heroes, 2 of which are simliar to Sans and Papyrus, that Gaster may be the result of Ninten's disappearance!
Ness would have known of Ninten, just like how Lucas knew of Ness. Which would explain how Sans thought that it wouldn't be worth your time to talk to Gaster. With all this together, we have a strong theory on just who Gaster really is, as well as Sans and Papyrus.
Ness fought for peace and justice, and what does Sans uphold? Justice.
Lucas fought because of his own insecurity. Which of the 3 is insecure most of the time? Papyrus.
Ninten was revered as the strongest PSI user and fought off a brutal enemy that betrayed those it held dearest. Which of these 3 is considered the strongest and fights you as if you betrayed everyone and killed them? Gaster.
I think this was more of a 3 for 1 theory.
While there's no question on jus who Sans and Papyrus are, Gaster still leaves a bit left to figure out.
The end of the first Mother only had him walk away and it was implied that he was never heard from again.
EarthBound mentions him as actually never being seen nor heard again after stopping the alien tyrant known as Gyiyg. (Don't ask me how to pronounce it)
So let's see, Ninten left and was never seen again, and then boom! We get Gaster?
It's possible that he also was trapped in time like Ness and that's how they both got there.
If so, this theory is all wrapped up. Little did we know that Gaster is actually the embodiment of Nintendo.
We now have a good idea on who Gaster is and we know what he says.
(Dio) Now we just need to figure out how he learned to speak in hands.
I'm still sticking with him becoming mute and learning sign langauge.