Shiki Tohno is Roa's 18th incarnation and can be considered "the real Shiki Tohno". This Shiki is not to be confused with Shiki Tohno, the protagonist of Tsukihime; though both of them are named Shiki, the kanji in their names are different. For the purpose of distinction, the real Shiki Tohno (Roa's current host) is often referred to as "SHIKI."
The real SHIKI Tohno was good friends with Shiki (an adopted member of the Tohno family from the Nanaya clan). SHIKI, however, went berserk after he lost control to his demonic blood, though Shiki initially believes it was because Roa took control of him at that moment. SHIKI nearly killed the adopted Shiki (who was defending Akiha), however Makihisa Tohno (SHIKI's father) forced the real SHIKI to the brink of death. He was then kept locked up in the Tohno Mansion's cellar for around eight years. Shiki's wounds nearly killed him, so Akiha gave him half her life, but, not completely dead himself, SHIKI used the Mystic Eyes that he gained from nearly dying and stole half of the borrowed life from Shiki, connecting the two Shikis, each possessing 1/4 a life, with Akiha having the other half.(Source: Wikipedia)