From Kirbypedia

Name:	Minimum
First Game:	Kirby & the Amazing Mirror
Enemies Copied from:	Minny
Helper:	None
Anime Episode(s)	None

One of Kirby's Copy abilities. Kirby shrinks to tiny proportions and uses his small size to scavenge for hidden secrets.

So far it has only appeared in Kirby & the Amazing Mirror, where it was needed to retrieve some treasure chests.

Games

Kirby & the Amazing Mirror

After copying a Minny, Kirby will shrink to its size. He retains the same controls, but loses the ability to slide, inhale, or float. Minimum has no attacks outside of ramming an enemy, which will also cause damage to Kirby and cause him to lose the ability.

Minimum is not very practical in many situations, and is only useful for crawling through small passageways, many of which containing treasure chests.

Trivia

Hidden in the code of Kirby's Adventure, sprites for a tiny version of Kirby can be found! Was this an early canceled ability?

Kirby Copy Abilities:

Cupid - Animal - Backdrop - Ball - Balloon - Baton Twirling - Beam - Bomb - Bubble - Burning - Clean - Cook - Copy - Crash - Cutter - Fighter - Fire - Freeze - Galaxia - Ghost - Hammer - Hi-Jump - Ice - Iron - Jet - Laser - Light - Magic - Metal - Mike - Minimum - Missile - Mirror - Needle - Ninja - Paint - Parasol - Plasma - Sleep - Smash - Smash Bros. Abilities - Spark - Stone - Suplex - Sword - Throw - Top - Tornado - UFO - Water - Wheel - Wing - Yo-yo

Final Weapons:

Star Rod - Love-Love Stick - Rainbow Sword - Star Chariot - Master - Triple Star - Block