Mini (also called Minimum) is one of Kirby's Copy abilities in Kirby & the Amazing Mirror. When Kirby inhales the miniature enemy, Minny, he too gets downsized to the size of a button. It is notable for being the only ability, besides Sleep, which actually hinders Kirby's combat skills rather than improve it.
Mini is only really useful to retrieve several of the game's treasure chests. This is because Mini has the ability to allow Kirby to enter small cracks and crevices. The only downside of the ability is that Kirby is a virtual sitting duck when such a small size, and has no way of defending himself against any foe or calling his friends with the cell phone. When Kirby goes through a door with this ability, he loses it. Even though this ability does not help with defense against enemies, it is a rare copy ability and near-impossible to find.
In Kirby: Squeak Squad, there are Kirby Bubbles that Kirby can find with a small Kirby-like figurine inside it. While it does not give him a power, he can get a 1-up if he mixes three of them together.
Although not an ability, Mini Kirby does make an appearance in the anime in which Kirby is shrunken down by a newly purchased Microtizer from Nightmare Enterprises.
In Kirby's Adventure, unused sprites for a smaller Kirby can be found. This set even comes with basic movement and an "ability presentation" sprite where Kirby shrinks. This may be lead to the point that Mini was once an ability for the game, but was removed from the final game. Ironically, since Kirby & the Amazing Mirror would be built off of the same engine as Kirby: Nightmare in Dream Land, the developers likely saw these lost sprites from the original game's code and revived the concept from there.