W. D. Gaster was the royal scientist before Alphys, responsible for creating the CORE. Gaster's Followers relay various explanations as to what happened to Gaster. One iteration says that he "fell into his creation," another says that "his experiments went wrong," and the last one says he "shattered across time and space." Characters such as the River Person and Gaster's Followers hint at his existence. Similar to Sans and Papyrus, he speaks in a typeface different from the other characters
His voice is very deep, sophisticated and is very raspy. His voice will also have a distortion filter.