Tohno SHIKI

The true eldest son of the Tohno family.
He was always pretty talkative and brash, but after Roa surfaced he got even chattier.
His having a different appearance in the Arcueid and Akiha routes is just a little bit of fan service.
After all, the only real difference between the two is whether or not Roa or SHIKI's consciousness is stronger, but we thought it would be pretty terrible if the player had to keep seeing the same face pop up over and over as the villain in a game as long as Tsukihime, so we gave them different sprites.
By the way, since Akiha is “Aki (Autumn),” his nickname among the staff is “Haruo (Spring Boy).”

Tsukihime Data Collection: Tsukihime Dictionary
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The true eldest son of the Tohno family.
He was always pretty talkative and brash, but after Roa surfaced he got even chattier. His having a different appearance in the Arcueid and Akiha routes is just a little bit of fan service. After all, the only real difference between the two is whether or not Roa or SHIKI's consciousness is stronger, but we thought it would be pretty terrible if player had to keep seeing the same face pop up over and over as the villain in a game as long as Tsukihime, so we gave them different sprites.
He is a handsome young man, casually dressed in traditional Japanese clothes. Somehow, he has the air of a haiku poet about him.
One of his abilities as a demon hybrid is a “death-resistant body.” Just as it sounds, it means he has a body that will not die. However, this is not the ability to heal wounds, but the ability to reconfigure his body in order to survive despite his wounds. It is this ability to freely manipulate his flesh that allows him to use his own spilled blood as a sword by coagulating it.
…He is yet one more hapless victim of the Infinite Reincarnator, Roa.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised