Mr. Mummbles is Ruby's elderly nextdoor neighbor. Mr. Mummbles lives in a glass house with his pet, a frog named Socrates. Mr. Mummbles received his named from Poe in the episode "Poe-Ranoia", where Poe thought that he was a murderer, and since then the name has stuck. After a revelation that Mr. Mummbles designs and creates amusement park rides, he can be seen in later episodes testing these new rides. His rides are designed to make people happy, sometimes having features that tickle the passengers. Mr. Mummbles also follows the rule "Safety First", however his rides seem to bend in the opposite direction to being safe, which includes massive spirals, gaps and fire on the tracks.

He is voiced by an uncredited Derek McGrath.

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