One

W.D. Gaster was a brilliant monster.
He had heard this his whole life; with every class he took, with every test he aced-someone had mentioned his intellect.
He had worked hard for this-especially from an early age when he had difficulty speaking and writing. He himself could understand what he was saying and could read his own writing perfectly; yet others couldn't understand him. Gaster spent almost a year constantly practicing and teaching himself so that others could understand him.
So when he had gotten an internship with the royal scientist he was thrilled that everything he worked for had paid off.
Once he had gotten the internship, it wasn't too long after that other scientists were talking about when he'd be a full time scientist. His mentor had even made a slightly joking comment about how Gaster would make him lose his job soon and replace him.
As his internship eventually drew close to an end, and the scientists got together the paperwork to officially extend him a job offer; the king caught wind of the promising young monster. When Asgore got the paperwork, he denied it almost instantly; writing in the comments that Gaster should enjoy being young while he still was and not jumping into the workplace before the internship was over.
But once the internship was officially over, the top scientists returned, anxiously asking the king to reconsider; Asgore, somewhat reluctantly, signed to let him become a member of the royal scientists.

And Gaster did not disappoint.
A war with humans was soon declared and Gaster jumped to help develop laser weapons to combat the fast losing war they were holding.
They lost, regardless.
It wasn't until everyone was forced into the underground; that he was given the title of the head royal scientist. It left a bad taste in Gaster's mouth.
He was instantly in charge of rounding up the other scientists and making use of whatever tools anyone had managed to grab to create new equipment from scratch. And of course, they ran into their biggest problem; power.
An entire race of monsters trapped underground with no access to anything besides what they had happened to have on them when they were exiled meant almost no technology.
They still had magic; Gaster couldn't count the times he was thankful for any monster with electrical magic that lent a hand to them. But that was tiring especially since the number of monsters that had electric based magic was limited.
Weeks passed, and Gaster was finally able to return to the king with a plan of action for energy; at the cost of a LOT of physical labor from monsters.
Asgore agreed on the spot, and assisted in overseeing and helping out with the project. The faster they had a source of power up and running, the better for everyone underground.
Thus, Gaster made blueprints upon revised blueprints for this project. A single power source capable of fueling everything they would possible need in the Underground.
The Core.