Ukiki
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Ukiki is a monkey in Super Mario 64 and only appears in the course Tall Tall Mountain. If approached, he'll steal Mario's hat and will show off. To retrieve the hat, Mario had to catch him.
If Mario loses the hat in the level then dies, go back to Ukiki, he'll be wearing Mario's Hat. In the second mission, Mario must catch him, and he will jump on a cage with a star for him. Just jump directly down with him and Mario will be able to access the star.
Ukiki also makes cameo appearances in other Mario titles, including Mario Party 4, in which the monkey-like creature will toss a banana. This banana, upon being slipped on, will determine a character's path around the game board.
A relative to Ukiki named Akiki appears in Mario Party Advance.