A secret character who is hinted by people like the River Person is W.D. Gaster. According to clues scattered throughout the game, he was the royal scientist before Alphys and was responsible for creating the core, but he fell into the creation (which is implied to be the core, but could be the strange machine in San's secret workshop) and his life was cut short, disappearing from existence, but his consciousness is still present. Other than minor hints, trying to find W.D. Gaster and direct references to him can't be found playing the game normally, but can only be found if you edit the game's files and fun values. Also, if you go to your save file by editing the game files as well, you could also get a message from W.D. Gaster himself in the WingDings font (which is who he is named after), which he talks about his research, and then the game will crash.