Ghidorah

"Accursed traitor! You dare ally yourself with the humans after turning your back on me?! Unforgivable! Absolutely unforgivable! I'll make an example out of you, Magara!"

- Ghidorah's grudge against Magara Ghidorah (ギドラ, Gidora) is an ancient Dragon and former ally of Magara, being friends prior to the Dragon Civil War, but became enemies after Magara opposed Ghidorah, who used the ongoing war between Humans and Dragon as an excuse to commit what is essentially genocide on both races, relishing in the chaos and violence, causing him to be a dreaded creature among both armies, granting him the titles of Murderer of Kin (殺人者の一門, Satsujinsha no Ichimon), but most commonly, is known as the Mephisto Dragon (メフィストドラゴン, Mefisuto Doragon), being comparable to a heartless demon in comparison to the rest of his kin.

Ghidorah was defeated by Magara, ripping his body to shreds, barely leaving anything behind. However, through sheer will and undying thirst for destruction and revenge, Ghidorah kept what was left of his body alive, hiding and encasing his body in stoned scales, essentially hibernating, until he was found centuries later by Hakase, using the same technology found on Jabberwock to cybernetically rebuild Ghidorah, awakening him once more, which predictably resulted in his escape from the council's grip, free to do as he pleases once more.

Trivia

 * Ghidorah is named after King Ghidorah, a three-headed dragon kaiju from the Godzilla franchise.