Kouma Kishima is the man who eliminated the Nanaya clan under orders of Makihisa Tohno. He is a member of the Kishima clan, a clan which is a sub-family of the Tohnos but with much stronger demon blood running through them.
Kouma himself, however, is a tranquil character, similar to a mountain ascetic aiming for enlightenment in order to find meaning in his own existence. As the Kishima have fallen into decline, Kouma would have been fine if he just quietly faded into obscurity in his forest den, letting the line die out for good.
Kiri Nanaya’s existence has affected Kouma a lot. Before his death match with Kiri, Kouma never felt the sensation of truly being alive until Kiri engrave the “sensation of being alive” into Kouma. But, because Kiri showed fierce resistance to the end and even caused serious wounds on Kouma and made him feel pain for the first time in his life, the mixed-blood recognized the demon-hunter as being the embodiment of Hell.
Kouma was born into the Kishima family as something that shouldn’t have been born. He is the eldest son of the Kishima. As a family of Mixed-Bloods, they continued to mix with Oni while attempting to make their bloodline stronger, and the sin of their several hundred years of work finally took form in their generation. He had preserved the desired purity they had been waiting for when he was born, but even they, who were not normal, realized that he was the path to their destruction. They thought that this is where the bloodlines of all hybrids end, and that they were either just terribly slower than the others or arrived there terribly early.
Calling him a Demon Child, they desperately tried to hide away his existence. He was given a dark, traditional Japanese-style room, and that was all the freedom he was allowed. He was a small child, so he was likely satisfied with the space. He would also often glare at the space as if it were too large, but he would have been unable to voice his complaints in the first place. He didn’t understand people, their behaviors, expressions, or words, or anything else very well. He thought that he must have been a dog rather than a human, and if that wasn’t the case, the severe gap must have meant he had gone crazy.
He was never taught anything, and as no one talked to him in the first place, there was no need to learn words or tell others his thoughts. His eyes could pull out the inner side of people. Most of the people who met him were nothing but terrified, so that is how he learned people were like that. Since they only acted like that, it was natural there was no need to learn words. He would have rather been among the animals in the forest he liked so much, but they couldn’t let the family’s successor, even if only in formality, move around in such a manner. It was likely an embarrassment for them to have the next head wish to live as a beast.
While wild, he wasn’t a fool like they so believed, and he matured very quickly. He learned how to seal his power early during his life, and he never once opened his cage since he was born. He kept suppressing his thirst and the beast called himself, much like a fasting that began right when he was born. It is normal that a hybrid only feels such bloodlust after growing up as a human and finding something in need of protection. The Inversion Impulse of his bloodline destroys a person within a year, even with the mind of an adult. He understood that fact quietly, even though he was a child without words or the concept of a family.
He was trying to protect his family even without knowing, even without understanding how such things as a father, mother, or siblings are related to him. He still suppressed himself to protect them, likely because he knew that returning to soil peacefully someday after being forgotten was the right thing. It didn’t last long, and the one who put an end to it was someone other than him. Receiving the curse, “You were out of your mind since you were born.”, he was shot in the head with a bullet. The words were true, as he was different since he was born, which is why he was able to suppress the blood longer and didn’t go crazy like them. The boy who had been out of his mind since he was born was unable to even die from such a thing as a bullet.
After the mansion burned down and the clan was slaughtered, only he was left. However, because the Kishima family itself has already died out, he is treated as the family head. It was easy for him to end something like a family because he was unable to understand it in the first place. He possessed no regret, emotion, remorse, or feelings, and he was a living being who could only exist that way from the beginning. The head of the Saiki family arrived afterward, and they eventually took him into their mansion. Barely ten years old, he was chained by both his arms and legs while being covered in wounds. He encountered Kiri Nanaya during his slaughter of the Saiki family, and he had his eye crushed due to Kiri’s unconscious idea that it would let him defeat Kouma later in life. His head simply dropped down without a scream, and Kiri left the room while the dripping blood stained the floor.
Due to Makihisa Tohno orders, he slaughtered the Nanaya clan. The last man standing was Kiri Nanaya and Kouma faced him to a death match, the gap between him and other humans was finally solved due to meeting the same type of person as him. Since Kiri was just a human, the outcome of the fight was decided from the beginning.
He spared the life of the young Shiki Nanaya due to having felt that the two would meet again in the future. After the Nanaya clan massacre, Kishima voluntarily distanced himself from human society and lived as a hermit in the mountains until 3 years prior to the events of Tsukihime, when he was allowed to reside in the Tohno mansion as the head of the Kishima family. Even while living in the Tohno mansion, he was rather quiet and distanced himself from the other residents. Kohaku notes that whenever she called him ‘young master’, he would give her a cold look and leave. When Akiha requested that he and the other residents leave the mansion (due to her calling Shiki back), he apparently went back to living in the mountains.