Ultimate Ghost is the main antagonist and final boss of Find Mii. He is the secondary mastermind behind the kidnapping of the monarch in Find Mii (the main mastermind being the Dark Lord.).
He first appears after the defeat of the Armored Fiend, where he reaches out with his hand and grabs the monarch.
He appears as a giant, purple demon with horns, yellow eyes, and a spiky mouth, when defeated, the Ultimate Ghost glows and fades away. However, he reappears in Find Mii 2. In Find Mii 2, it is revealed that he is not the actual leader of the monsters, and the Dark Lord is revealed to be the true leader. In your encounter with the Ultimate Ghost in Find Mii 2, he is more powerful, see stats for more.
Heroes have a harder time landing Sword Attacks while facing the Ultimate Ghost. After his defeat in Find Mii 2, he vanishes once more and is never seen again.