Fuma Kotaro. It is suspected that his true form, the suspicions continuing on for generations, was an Oni or an existence that was comparable to it. And in addition, it is probably a being with the stature of a person from a foreign country with rough seas and the like. His reasoning is equipped with Mad Enhancement. All parameters improve to the extremes, and his stamina is recovered every turn. As compensation, a portion of his being starts breaking down due to outputting his brain beyond its limits (overdrive), severe damage is done to his internal organs and lifespan, and so forth.
It is unavailable to him due to it being sealed, but Kotaro thinks studying Oni Kinds could potentially allow him to release it.