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 says that he "shattered across space and time." 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 other characters.

Gaster is a very mysterious character. Most info on him is fanon, so there's nothing really to go off. So, to make it easier, I'll say that gaster would sound similar to Jordan Woollen's representation, linked HERE https://www.youtube.com/watch?v=TLH398sxYdU