W.D. Gaster was the Royal Scientist before Alphys. The only way to find out about him in-game is through FUN events and hacking your room number. According to one of his followers, he created the CORE, but then his life was cut short and he fell into his creation. It is never specified what creation he fell into, however it is implied to be the CORE.
Another way to encounter W.D. Gaster is by changing the room number in the undertale.ini, file0, and file9 files from the Undertale files to 264. This brings you to a room named room_gaster, in which some sort of entry is detailed. This entry says:
“ENTRY NUMBER SEVENTEEN
DARK
DARKER
YET DARKER
THE DARKNESS KEEPS GROWING
THE SHADOWS CUTTING DEEPER
PHOTON READINGS NEGATIVE
THIS NEXT EXPERIMENT
SEEMS
VERY
VERY
INTERESTING
...
WHAT DO YOU TWO THINK"
After this, the game closes without explanation.