Zoldeo
Everything happened with a reason, so when Zoldeo had possessed Capricorn's body, his lust for power hadn't just sprung up out of nowhere. Nay, it had come from his fascination with Celestial Spirit Magic.
The power to summon a familiar to your aid had been one of the most incredible things to Zoldeo. In fact, he began to desire the power to summon more than just some tacky-looking Celestial Spirits. Even before Layla had retired and entrusted him with Capricorn, Zoldeo had been searching for a way to go beyond summoning mere Celestial Spirits and start summoning humans instead.
Zoldeo knew that the amount of magic in the world was nearly infinite, and that there must be a type that allowed him the power that he was looking for, and he eventually found it. The Lost Magic known as Human Subordination. It was the perfect magic for Zoldeo's tastes, and as he had desired, he learned the magic and gained the power to bend any human to his will.
Then Capricorn had to go and spoil his fun by trying to "advise" him. But Zoldeo would have none of it however, and still on his high from obtaining a powerful magic, Zoldeo tried to impose his will on Capricorn only for him to possess the goat spirit instead.
In retrospect, it had been quite foolish to try and use a magic meant to subdue humans to try and subdue something that wasn't human at all. He was now forever stuck with the body of a goat instead of his own. But Zoldeo didn't care about that. He would find a way to gain a human body through any means possible, even if it took forever.
He had quickly set to work, trying to find a way to rid himself of Capricorn's body and regain a human one, all the while using the name Caprico. Along the way, he kidnapped and subjugated several prominent figures with his magic, turning them into his pawns. He even took two mages, Yomazu and Kawazu, under his proverbial wing, making them his disciples.
He searched far and wide for years, falling more and more into despair as he began to come to believe that maybe he would never be able to regain a human form. Even amidst his self-pity, he found his situation strangely ironic. He had become a Celestial Spirit with the power to summon human familiars. A reverse Celestial Spirit Mage so to speak.
Just as he was about to end his quest and quite possibly his life along with it, Hades appeared before him. It was like Zoldeo was looking at the devil when he met him. He was dark and powerful, the two things that made Zoldeo want to crawl in a hole and never come out again. Hades then invited him into his guild, Grimoire Heart, to become one of his Seven Kin. Zoldeo questioned as to why he should join, and was given a good answer: he would be able to regain his human body once again when they reached the Ultimate World of Magic.
That was all the reason that Zoldeo had needed to join. And so he did, becoming one of the dark guild's strongest members. He knew that once Grimoire Heart reached the Ultimate World of Magic, he would be free of that goat's body forever.
