Demon Hybrid

Refers to the descendants of humans who consorted with something “inhuman” in the distant past. Tohno Akiha and Kishima Kouma fall under this classification.
Though their blood is growing thinner with each successive generation, on rare occasions a child is born who has strongly inherited that “inhuman” characteristic.
As for Tohno Akiha, her blood is primarily human, but what inhuman blood she does have is exceptionally high class. Because of this, she could be considered the answer to the demon hybrids' evolution, capable of awakening as a demon hybrid while still remaining a human being.
Kouma, on the other hand, is an archetypical demon hybrid whose oni blood runs too thick, making life as a human completely impossible.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary