Nanaya Shiki

Eldest son of the Nanaya, a clan of demon hunters that passed down psychic power as their genetic inheritance.
The Nanaya had retired from the demon hunting trade to live a quiet life in the mountains. However, because Tohno Makihisa still regarded them as dangerous, they were raided and completely wiped out.
Shiki, too, was very nearly killed by Makihisa, but somehow he narrowly escaped death and was taken in by the Tohno family as an adopted child.
Though Shiki doesn't remember it, he had received basic training before Makihisa's attack.
This training was responsible for turning Shiki into a transcendent killer every time he was backed into a corner during Tsukihime… somehow.

Tsukihime Data Collection: Tsukihime Dictionary
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Shiki's former name, but in Tsukihime it is often used to refer to the manifestation of Shiki's murderous alternate personality. Especially in Kagetsu Tohya, in which we can see Nanaya surfacing of his own will and taking over Shiki's body… although this is somewhat the fault of Shiki himself. As far as Shiki is concerned, Nanaya is none other than the harbinger of Shiki's destruction.

MB Official Strategy Guide: Melty Blood Glossary
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Eldest son of the Nanaya, a clan of demon hunters that passed down psychic power as their genetic inheritance.
The Nanaya had retired from the demon hunting trade to live a quiet life in the mountains. However, because Tohno Makihisa still regarded them as dangerous, they were raided and completely wiped out.
Shiki, too, was very nearly killed by Makihisa, but somehow he narrowly escaped death and was taken in by the Tohno family as an adopted child.
Though Shiki himself does not remember anything from when he lived in the Nanaya village, he did receive basic training before Makihisa's attack. This training was responsible for turning Shiki into a transcendent killer every time he was backed into a corner during Tsukihime… somehow.
In Melty Blood, Nanaya Shiki appears as Tohno Shiki's anxieties given form. He uses the Nanaya martial arts that Shiki should have learned, but unlike Shiki he does not possess the mystic eyes of death perception. According to Miyako, he is her “wild big brother.” Also, perhaps it is due to Wallachia's influence, but he is prone to somewhat theatrical elocution.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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Shiki the Killer. Eldest son of the Nanaya, a clan of demon hunters that passed down unique abilities as their genetic inheritance.
Though the Nanaya were once feared as the natural enemy of demon hybrids, the last family head, Nanaya Kiri, retired from the demon hunting trade to live a quiet life in the mountains. However, because Tohno Makihisa still regarded them as dangerous, they were raided and completely wiped out.
Except, that is, for Nanaya Shiki. Even though the Nanaya bloodline should have been annihilated without exception. Even though he was the family head's only son. Somehow, for some reason, he alone survived.
Unlike Tohno Shiki, he uses the Nanaya martial arts, which strains the human body to its utmost limits, without any fear or hesitation.
Also, this is just a digression, but in the game when he comments on the lack of skill after winning a battle, he's actually talking about himself and not the opponent. After all, an assassination is something that's over in an instant. For it to turn into a drawn-out “battle” against an enemy, well, what can he do but laugh at his own failure?

MB Act Cadenza PS2 Manual: Tsukihime Dictionary