This item is just a three-dimensional "?" that is deep blue and somewhat speckled. Normally appears in a Giant Bubble, but can be seen outside the bubble in the Plaza or if you pop the bubble with a Package or similar item.
When obtained, a random item effect will be chosen and applied. These items only appear when you "Taunt" the Balloon Fish by flying down by the water and avoiding being eaten. As such, these items are very useful and easy to get if you understand the timing. By getting multiple items this way, the game can become far easier to complete, as you can layer the effects of all the items, such as invincibility or slowing down time.