In the intro to Kirby and the Amazing Mirror, you see Meta Knight fight his evil clone, Dark Meta Knight, in the Mirror Word. Meta Knight successfully defeats Dark Meta Knight and imprisons him in the Amazing Mirror that acts as a passage between Dream Land and the Mirror World. Cut to Kirby minding his own business, when suddenly Meta Knight appears out of nowhere and attacks Kirby, splitting him into four! That's a bit out of character for him, but you pay it no mind as you're soon thrust into the Mirror World, looking for the pieces of the Amazing Mirror so you can return to Dream Land. The way forward, however, is never what it seems to be, and to reach the Mirror Shards, you have to search every nook and cranny for secrets and hidden passages. So eventually you face Meta Knight, who naturally doesn't want you to restore the Mirror since it would free Dark Meta Knight, and... Wait, what's this? He's not giving you a sword? That's definitely out of character. Something fishy is going on here... So you find the remaining pieces of the Mirror, and finally come face to face with Meta Knight inside... And then the real Meta Knight appears! The Meta Knight you've been fighting is actually Dark Meta Knight, and the Meta Knight that was imprisoned was the real one all along! After knocking the crap out of Dark Meta Knight, he reveals his true form: an evil sorcerer known as Dark Mind with an appearance similar to Nightmare. Look closely, however, and you'll see a fiery eyeball where his torso should be. Strange... After defeating Dark Mind a few times in a row, he then reveals his actual true form — the same fiery eyeball that he had for a torso, only much larger! Even after you finally reveal his true self, he still tries to throw you off with tricky tactics, such as reflecting his shots off of mirrors and even flipping the screen upside-down! So after you finally defeat Dark Mind, he explodes as the four Kirbies fly off back toward Dream Land. ...And then it's revealed that the Shadow Kirby enemy you've been beating up the whole game is not a bad guy like you probably assumed, but is actually the hero of the Mirror World! Every aspect of the game, from the story, to the presentation, to the Metroidvania gameplay, is representative of a central theme: Kirby and the Amazing Mirror is a game about exposing lies.
Except that the opening cutscene made it pretty clear that Dark Meta Knight beat Meta Knight, not the other way around. Also, I think it's generally assumed that Dark Meta Knight was a minion of Dark Mind, not a shapeshifted form or disguise, but the game itself doesn't give any clear information on that issue one way or the other (while Dark Meta Knight is apparently Killed Off for Real after his second defeat, Dark Mind turns into a mirror and shatters in a similar way after his first four defeats, making it a bit more ambiguous), so I'll let it slide. Otherwise, a solid Fridge entry. Dark Mind's a Giant Space Flea from Nowhere either way.
Dark Mind's design makes a lot of sense when you break it down. Aside from partially resembling Nightmare, it's torso area appears to be partially mechanical. It's possible that Dark Mind is either bio-mechanical or that it's body is a machine powered by it's true form (whose eye has cybertronic patterns, which are absent in it's final form). Further still, if Team Kirby Clash Deluxe is any indication, Dark Mind would have access to a hefty amount of technology due to Parallel Susie's ties to him.
When you think about it, Kirby really didn't accomplish anything in this game. Kirby: Triple Deluxe revealed that Dark Meta Knight survived the events of the game, and Team Kirby Clash Deluxe showed that not only did Dark Mind survive (even after that six-phase beatdown), he proceeded to possess Shadow Dedede to form King D-Mind, and now has Dark Taranza and Parallel Susie as his minions. If anything, Kirby only made things worse because Dark Meta Knight's rage at being defeated in battle caused the Dimensional Mirror to become corrupted, driving Sectonia insane and kickstarting the events of Triple Deluxe. Not to mention, while Dark Meta Knight and King D-Mind are wreaking havoc, Shadow Kirby is nowhere to be seen.