Ouki

Ouki (凹気; Lit. Hollow Soul), is a Revenant brought back to life by an attempt to use Edo Tensei. Though the technique succeeded in bringing him back to life, it corrupted his physical form, turning him into a being composed mostly of bone, but with all seething hatred and contempt. Unable to control him, Ouki killed his summoner, breaking the contract of his summoning, allowing him to roam free. Before his revival, Ouki was a man who was at the forefront of genetic studies, a thing once considered heretical over eight hundred years ago. His studies led to forefronts in the creation of powerful elements, things like the illustrious Wood Release. Creating a prototype form of the Kekkei Genkai, Ouki administered its usage in many wars, using its power to turn the tides in combat. But he became less and less sane over the years. After twenty years of genetic experimentation, his life was cut short when he hung himself in his laboratory. Though he was well known during his own time, his reputation faded away as the centuries passed, until he was nothing more than a nameless figure.