Cuber

Cuber narrated the beginning and end of the episode, "Five Short Graybles." He has a large head and wears a hat with a floating cube on it. His body is much smaller than his head. He seems to wear some kind of advanced material. He seems to know Finn, Jake, BMO, and Princess Bubblegum but calls Ice King "the blue guy" and Lumpy Space Princess "the other thing." Based on how he spoke he may be from the future past Finn and Jake, considering he says the stories seen in "Five Short Graybles" are tales from "the days of old." Also it appears as though he lives in the Crystal Dimension considering what is in the title card, and the fact that he is using crystals as wood in his fireplace.