Cuber

Cuber speaks in the beginning and end of the episode, "Five Short Graybles." He seems to know Finn, Jake, BMO, and Princess Bubblegum but calls Ice King "the blue guy" and Lumpy Space Princess "the other thing." He may be from the future, considering that he says the stories seen in "Five Short Graybles" are tales from "the days of old." This can be supported by the fact that his clothes and room look advanced, though similar technology can sometimes be found in Ooo. He uses a variety of non-English terms and appears to have a heavy accent of some sort (see quotes).

A promo for season 5 shows that he will return in one new episode, which is "Another Five Short Graybles."

Appearance
Cuber wears a hat with a small floating purple cube next to it, the hat itself is a hemisphere with a bent line coming from the top. The hat is the same color as his robe, which is light salmon colored. His robe has a green, blue, red, and translucent orange squares on it. Its chest has a line with many 90 degree angles dividing it approximately down the middle. The line ends abruptly suggesting that the top and bottom halves of his robe could be separate. His skin is light pink, and his head is significantly larger than his body. His features are similarly sized to most characters but spread to fit his face, giving him a comically small mouth.

Quotes
"Hmm? Oh hoho gleeb old bayble grapes, you're just in time. Bayble Cuber's bound to watch an incredrible [sic] adventure from days of old, on my Holo-Pyramid Viewer. 5 stories - each relating to one special theme. See if you Graybles can guess the theme by the end of the episode, OK? Uh sit back, relax, and enjoy the show."

"What? What's that? You haven't guessed it? Oh, I'm sorry. Well, pause your viewer if you want to think about it before I belable it to you ... The theme was the five senses, hah, you see! Look: BMO SAW his reflection, Finn and Jake TOUCHED their hands together, the princess made a TASTY sandwich, the blue guy SMELLED bad, and the other thing HEARD music."

"Ho ho, what a treat! OK well I'll see you crimpy glimmers on triode flimpin' the diode, ah ha ha hmm! (waves)"

"Now you've got it, I can tell. The theme is the Five tastes. You see, Jake ate a SWEET plum. Marceline played some SAVORY licks. Tree Trunks have a SOUR taste in her mouth, Oh my. Gunter wanted to taste the SALTY tears and Football's tea was BITTER."

Trivia

 * The title card of "Five Short Graybles" features a giant crystalline structure in space with Cuber's silhouette appearing in its window. The title card also features Finn and Jake however, which contradicts his suggested place in Ooo's timeline. Title cards often depict scenarios that do not actually happen in the episode, so it is unknown whether any part of this image can be taken as fact.
 * The hologram of the Earth in Cuber's room on the right side shows the crater presumably created by the Mushroom War.
 * He is the only character to break the fourth wall by directly addressing them so far, as he addresses the audience by calling them "Baybles."