gerson
A character from Undertale, one of the shopkeepers and the elderly mentor/idol of Undyne.
He is also mentioned in Deltarune with full name of Gerson Boom, deceased author famous for the book series "Lord of the Hammer" and is Father Alvin's father.
He appears in Deltarune Chapter 4, revived from his ashes through the power of a unique Dark Fountain. Here, Gerson is simply referred to as an "old man," with his name only being confirmed in the chapter's epilogue. He serves as the secret boss and Shadow Crystal holder of the same chapter, under the title "Hammer of Justice."