Gumbald

Gumbald is Princess Bubblegum's great-uncle. Although he has not yet appeared in an episode, Princess Bubblegum mentioned him in "Susan Strong," claiming that he had cut down an entire field of taffy trees in a single night; the reason for which is unknown. Finn and Jake were supposed to pull out all the tree stumps that he left behind, but only pulled out one before encountering a metallic hatch that would lead them to Susan Strong.

Trivia

 * So far, he is the only known relative of Princess Bubblegum that hasn't been "created" by her.