From the Super Mario Wiki
Tumble is a "magical die" that the Millennium Star brought to life that replaces Toad as the host of Mario Party 3. His appearance is actually very similar to that of a Toad, but he wears gloves and has a die on his head instead of a mushroom cap, as well as only having hands and feet.
In addition to explaining minigames and controls and getting a Battle Royal or Duel Board started, Tumble will also show the players the new location of the Millennium Star each time it is moved. He (along with the Millennium Star himself) also gives out Star Stamps whenever a player has won at a board.
At the end of the game, the top of Tumble's dice cap opens to reveal the real Millennium Star.
Tumble re-appears as the host of the Shroom City mode in Mario Party Advance. He is also said to be one of Shroom City's guides; he kept this role by the end of the game (which describes him as a "beloved Shroom guide"). Tumble can appear by landing on a Mushroom Space, and gives the player two mushrooms if they win the minigame.
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