Mumbo is a shapeshifter, but he has trouble controlling his abilities (s8 was him trying to learn how to control them better, but that backfired).
He was raised in a cottage in the forest with his siblings, Lizzie and Jimmy (spreading my failblings propaganda >:3) far away from society so he doesn't really understand how to be a proper "human". He hid his vampire traits until s7 when they were accidentally revealed, and then revealed he was a shapeshifter in s8. Shapeshifters originally were created to copy humans and sneak into their villages to steal food and shelter (kind of like how some birds will lay their eggs in another bird's nest), so Mumbo has an inate sense to copy the hermits around him, which is why he learns to build using what other hermits have created. He also picks up on some mannerisms that other hermits have, such as Grian honking like a goose when he's upset. Its just him trying to be "normal" in a group of the most un-normal people in the world.
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