Mirror is one of Kirby's Copy abilities. It debuted alongside abilities like Yo-yo and Plasma in Kirby Super Star. With this illusory ability, Kirby is granted the power to create mirror images of himself to confuse enemies and perform multiple attacks at once. This mind boggling ability also gives Kirby a golden scepter which is able to project a glassy, near-invisible blade which can not only be used as a sharp blade to slice enemies, but also as a shield to protect him from enemies too close for comfort and their projectiles. This is very helpful against bosses that attack with projectiles that cannot be blocked (namely Computer Virus).
Not only does Kirby get a magic scepter when he inhales the wizardly Simirror, but also a jester cap that looks just like the hat Beam Kirby wears. Strangely, the coloration of this hat is also reminiscent of the hat Marx wears (apart from the fact that Marx's hat colors are switched, this actually makes sense considering the fact that it is a mirror, which would mean flipping the image).
Mirror is mostly unchanged from the game version in the anime, and Kirby wears the same hat. The attacks Kirby uses are Reflect Guard, Mirror Body and Mirror Cut. When he uses Reflect Guard, he gains a translucent rainbow-colored force shield. Mirror Body is greatly updated - instead of only two, Kirby creates dozens and dozens of copies, all of which can harm the enemy. His most powerful is the Mirror Cut, where his scepter turns into a shimmering sword that he uses to cut the enemy in two.
In the transformation sequence, Kirby looks at his reflection in a mirror then spins until it breaks, turning into Mirror Kirby. The mirror shards form into the tip of his scepter.
Some of Marx/Soul's attacks involve him splitting in half and doing a certain attack. One of the moves Kirby can use with the Mirror ability lets Kirby split in two and damage every enemy that comes in contact with Kirby. Because of this, Marx's mirror-like wings, and the fact that Marx's hat resembles that of Mirror Kirby, Marx may be related to this ability.
Like the Beam ability, the official artwork shows stars on both sides of his hat.
In the anime, Mirror Kirby had two versions of transformations. The first one that appears in the episode One Crazy Knight had a basic normal looking transformation and the one that appears in the episode Buccaneer Birdy had the same animated transformation but with added 2D and 3D special effects.