Meta Knight
The two Meta Knights fought each other until the real Meta Knight is defeated. He is then knocked into the mirror, which is then cut into eight fragments by Dark Meta Knight (which are then scattered across the Mirror World), so the Kirbies must save Meta Knight and the Mirror World. After collecting all eight mirror fragments, Kirby enters the Mirror World and battles Dark Meta Knight. After defeating him, a vortex appears and sucks the Kirbies in, who is given Meta Knight's sword. The Kirbies then proceeds to fight Dark Mind, the true mastermind behind the Mirror World's corruption, multiple times. Upon defeat, the Mirror World is saved, and Shadow Kirby (the Mirror World counterpart of Kirby who is believed to be an enemy, now ally) waves his goodbyes to the four Kirbies as they all exit the Mirror World via mirror door.