Gumbald

 Uncle Gumbald is Princess Bubblegum's great uncle. He has never appeared in an episode yet, but Princess Bubblegum mentioned him in the episode "Susan Strong". Princess Bubblegum said that he had cut down an entire field of taffy trees in a single night. The reason(s) is still unknown, Finn suspected that the trees were evil, but PB denied. In the TV Guide episode listing for the episode Too Young, it said her uncle tries to claim the throne; this could possibly be him, so he may not be deceased or it's a differnt uncle.