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He first appears in the Notes from the Underground episodes. Initially he was seen as the rock-like mutant that accompanied Quarry and Razorfist in their quest with the Turtles, and was captured by the Entity and turned back into his human form. However, like all the other Underground Monsters, his cure was conditional, and he was forced to live in the underground city in order to maintain his sanity and human form.
In Return to the Underground, it is revealed that he has reverted back to his monstrous form due to the loss of the Crystal Moon. However, Donatello is able to formulate a cure for the afflicted mutants, and he is able to return to the surface, fully human once again.