Cuber

Cuber is a character who speaks in the beginning and end of the episodes "Five Short Graybles," "Five More Short Graybles," and "Another Five More Short Graybles." He seems to know Finn, Jake, BMO, Tree Trunks, Marceline, Gunter, and Princess Bubblegum but calls Ice King "the blue guy" and Lumpy Space Princess "the other thing." As revealed in "Graybles 1000+," Cuber is an alien living 1000 years in the future, which was first hinted at when he said the stories seen in "Five Short Graybles" are tales from "the days of old."