W.D. Gaster (born August 27th) is a main character in MWSIH. He currently lives in a fictional town in Northern California.
Lucida - Lucida is Gaster's wife. She had died but was brought back to life by AskFriskAndCompany's Gaster.
Sans - Sans is Gaster's older son.
Papyrus - Papyrus is Gaster's younger son.
Frisk - Gaster acts as a fatherly figure towards Frisk.
Harlow - Harlow, or the Riverperson, is Gaster's younger sibling.