The king had just hired the new royal scientist. He was impressed by her ability to give a robot a soul. Alphys is her name. She would have a lot to live up to to follow up the previous royal scientist. The previous royal scientist had made tons of strides to the Underground, but nobody really knows who he was.
If you were to ask, the monsters would tell you that there was a previous royal scientist, but they couldn't tell you anything about him. They couldn't tell you what he did even though there were remains of his previous work. After all, the core didn't just pop up out of nowhere.
When Alphys moved into the Hotland lab, she did find scattered blueprints and abandoned machines. She even found a log from the previous scientist. It was written in a language she couldn't translate, but something about it seemed familiar. Even the hidden lab gave her a sense of deja vu she couldn't seem to shake. Everything about this was really creepy. She decided she would keep this hidden lab a secret for now.
If you were to ask the old scientist's followers, they would be able to tell you a little bit more. They would tell you that the previous scientist invented the Core. They would also tell you it took the king a very long time to hire a new royal scientist. They would say that the previous scientist met his doom when he fell into his creation. But he is not dead. He is always listening. They could even tell you his name. W.D. Gaster. These followers are also very hard to find, so you would likely never get the chance to ask any of them about Gaster.
The king was very aware that there was a royal scientist before. After all, there was a position open for Alphys. That is as much as he could tell you if you asked. Asgore had no idea that the royal scientist was his best friend, and that they had known each other as kids. They fought alongside each other during the War. His ex wife was best friends with the scientist's wife. But she too is gone in a long forgotten grave. Just another person Asgore had lost in his life.
The riverperson would tell tales, advice, and just plain nonsense if you got on his boat to travel between the different locations Underground. He would just say some silly things for the most part like joke about Temmie village. But sometimes he could say really chilling things.
"Beware the man who speaks in hands."
After hearing the news of the new royal scientist, Sans fell even more into the dumps than usual. Sans just hadn't been quite the same since dad disappeared. No one seemed to remember his dad at all. Not even his little brother. Sans could feel his memory slipping about his dad too at times, but he couldn't let that happen. Sans had a secret workshop that nobody but him knew about. His dad used to do work in there when he wasn't in the lab. Now Sans kept the place locked up tight. There were blueprints, an unknown machine, a badge, and a child's drawing in there. Right after dad disappeared, Sans panicked and tried to get the machine working to see if he could bring his dad back. He never could. Now he just keeps it there to make sure he remembers his dad.
The drawing was made by his younger brother, Papyrus, when his dad was still around. It reminded Sans of better times. The drawing was of a younger Sans, Papyrus, a young human, and their dad. When Sans realized he was forgetting his dad, he took a sharpie and scribbled down the words "don't forget" on the drawing.
Sans took a deep breath and sighed. His dad was gone but Papyrus was still around and didn't seem to be going anywhere. Sans wouldn't let anything happen to his brother. Papyrus recently had decided he wanted to join the Royal Guard and was starting training with Undyne soon. Papyrus had somehow even looped Sans into being a sentry. It wasn't too bad as he got to take a lot of naps which his brother was not pleased about at all.
Sans put the drawing down and left the workshop. He could use some slacking off at Grillby's right about now. Maybe, Papyrus would even come drag him out if he was taking too long of a break. Sans smiled a little. His brother was just the coolest.
Deep inside the Void that he had created, Gaster had been contemplating. He hadn't been able to see his children in years. He was hoping he would be able to see his wife again if this all worked out but it hadn't worked. Now his soul had been split and he was being pulled through different moments in time. He no longer existed. He had seen that Alphys had become the new royal scientist and that made him happy. He knew his old intern would do a fantastic job.
Gaster feels his body being pulled again. This time he was being pulled further than he had ever been pulled before. That's when Gaster's whole life started to flash before his very eyes. His memories began to flood his vision, and then he saw nothing.