Gerson is a vendor found in Waterfall. His shop is located in a small cave east of the houses of Napstablook, Mettaton and Undyne as well as north of the river where the River Person is located.

An elderly looking olive green tortoise-monster, Gerson wears archeologist attire, with a pointed beard, a large magnifying glass, and a tan pith helmet. His teeth are crooked and yellow and his shell is a dark brown color with a pale cream rim. He also seems to be unable to open his right eye.

Due to living for a long period of time compared to other monsters and from studying history, Gerson has gained many knowledgeable insights about the history of monsters in the Underground, including the Delta Rune prophecy. Despite this, he seems to have trouble remembering certain information such as the location of Temmie Village and why Asgore was nicknamed "Fluffybuns." He also does not want to repeat the legend of the prophecy again if the protagonist asks him to. He was alive when Toriel also ruled the Underground with Asgore and while stating that it was tragic that she left, there was a bit of relief due to them being open to their affection to one another.