Frisk first heard about Dr. Gaster after their fifty-third reset. They knew it was their fifty-third because after their second reset they had started keeping a log of all patterns and irregularities.
Things people seemed to remember, calls they sometimes got and sometimes didn't, people that weren't always there - it all went into the log. The log was on paper some of the time. Most of the time it was in their head, the most reliable place they had to keep such a thing. It was easier to look for correlations if it was on paper, however, so sometimes they wrote it down.
That's how they knew they encountered the man on their fifty-third reset. They encountered him in Hotland, just outside an elevator. He looked gray, and stiff, and as if he didn't belong where he was right now. It was hard to imagine him as belonging anywhere. He stared straight ahead, and didn't give any indication of noticing Frisk. He looked fake, stuck on top of the environment. It took Frisk a few seconds to notice he had no shadow.
Only when Frisk greeted him did he begin to speak. He didn't turn his body, or his head, or his eyes. For all Frisk knew it might be a coincidence he started talking at that moment, while they were there. He talked about the old Royal Scientist, Dr. Gaster. That he created the CORE. That his life was cut short when he fell into his creation. That he wondered if Alphys would end up the same way.
That's all Frisk got out of him. He didn't respond to anything. Frisk didn't try touching him. They were afraid their hand would pass right through him.
Frisk spent a lot of time looking for more information about Dr. Gaster. There was nothing about him in the CORE. There were no clues in Sans's hidden lab. No hints on the ferry, or none that they understood.
"Can you find what you're looking for?"
Frisk looked up from the book they were skimming, at Lady Garf (age 46, born in Waterfall, moved to Snowdin as a teenager, picked up a word-search habit and a passion for cigars, visits Waterfall once a week to collect stamps).
"I have read every book in this library. What do you want to know? I can help you look."
Frisk had read every book in the library too. It wasn't very hard. Almost all of them were written by monsters, and the longest monster-written book was just twenty pages. But it had helped them before, and even if they didn't remember any book mentioning Dr. Gaster it didn't hurt to double check. They had all the time in the world. "I'm looking for something about Dr. Gaster."
"Dr. Gaster? He made the CORE, right?" She shook her head. "I don't think anyone wrote a book about him. Try asking around."
Frisk had assumed it was a mystery. The possibility of just asking people hadn't occurred to them.
As it turned out, Dr. Gaster really was a mystery, but he was a public one.
Asking in Snowdin didn't get Frisk far. A slime told them his initials were W. D., but didn't know what they stood for. At Grillby's Frisk heard he went missing a long time ago.
Waterfall was more fruitful. Someone with a clam for a head knew what he looked like, and gave Frisk a description. That led them to Napstablook. Napstablook had been around for an amazingly long time, and had met lots of people, but suffered from a bad memory for faces, names, and most other identifying characteristics. Frisk carefully described what Dr. Gaster looked like.
"Oh… him…" The ghost thought for a long time. "Yeah, I think he used to come here. He liked to feed the snails."
Napstablook thought some more.
"He always had a full bag of lettuce. Sometimes it took three hours… He was very patient." Frisk waited. "Um. I think that's all. I hope it helps…"
Gerson knew more. "Good ol' W. D.? Ha! Yeah, I knew him. Interesting fellow. Made weird tea. He wrote down the recipe once, but I can't find it anywhere. I've looked."
Frisk pried further.
"Fell into the CORE? Nah. Well, he could have, but people just made that up after he disappeared. They've been joking about guard rails for decades, see?" He scratched his chin. "I don't know what happened to him. Lemme know if you find out. I miss him."
Hotland was next, and first in Hotland, Alphys. She started chattering excitedly when she heard the word "Gaster."
"Oh! Yes! I know about this! He used to be the Royal Scientist! And then he disappeared! And people say he fell into the CORE but nobody really knows because he didn't leave anything behind! Like, literally! All his stuff is gone! Even his house! It's like it was never there!"
She breathed in deeply.
"Sorry. I'm just really into this, you know? It's super strange! It's, uh, one of the reasons I wanted this job. To look for clues. But there aren't any. People still remember him, but it's as if he never existed."
She hesitated.
"You know, some people on UnderNet think he didn't. That he was never real, and that he's a fake memory. But we just don't know! Nobody knows! It's so weird! Let me show you the wiki…"
Most people who knew Gaster - or thought they knew him, whichever it was - lived in New Home. Frisk didn't like going there. They had been recognized a few times by monsters old enough to know what humans look like, but not wise enough to ignore them. When that happened they had to go back and try again. It was not dangerous, not really, but it was annoying.
Frisk met a few of the people who built the CORE. Even the CORE's true designer, who didn't remember designing it and only knew about designing it because it said so in all the records and was constantly apologetic about it.
They met one of his schoolmates, who told them about Gaster's interest in group theory.
They visited the place where he used to lunch most afternoons. He liked to order dishes with tomatoes. He had a keen interest in hydroponics.
They visited the street where he used to live, and looked at the narrow space between number 64 and number 66. The people who lived at number 66 at the time thought they lived at number 68, and the people who lived at number 68 thought they lived at number 70, and so on.
They visited the mailman assigned to that street at the time, who shuddered when they brought it up.
Gaster had been married. Frisk didn't manage to get in touch.
They ran into Asgore. He stammered something about ferns, trying very hard to hide that he recognized them.
Near the center of New Home Frisk found a small building with a shrine. There was a portrait, drawn from memory. A pen of the brand Gaster swore by. A special kind of ball joint, designed for the CORE. A healthy potted fern. And much more.
There was also a plaque. It didn't have a name, or a date. It just said "People aren't really gone as long as we remember them."
