Inui Arihiko

Shiki's partner in crime.
He's been hanging out with Shiki since elementary school, and had the same antisocial attitude and style of dress even then. His parents died in a disaster when he was young, and he was more-or-less raised by his older sister.
As for why he and Shiki are friends, well, there's a very good reason for it but it's a story for another time.
The second Arihiko laid eyes on Shiki, he intuitively knew that he was broken. “I've decided, you're going to be my rival.” So he declared, while eating the pudding he had just stolen from Shiki's lunch.
By the way, during the initial design phase, Arihiko had a character sprite completely devoid of any sort of “bite.” The scenario director absolutely hated it. So, several days later the art director came back, asked “Fine, then how about this!” and whipped out Arihiko's current design. The scenario director couldn't help but burst out laughing. He must have loved it so much his emotions just up and exploded, yeah.

Tsukihime Data Collection: Tsukihime Dictionary
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Shiki's partner in crime. He has a tendency to wander.
He's been hanging out with Shiki since elementary school, and had the same antisocial attitude and style of dress even then. His parents died in a disaster when he was young, and he was more-or-less raised by his older sister. He's a model outlaw (huh?), and enjoys living a delinquent lifestyle that doesn't bother normal, upright people. In contrast to Shiki, who became an outcast for the sake of those around him, Arihiko did it for himself. Though he's rumored to be something of a womanizer, his exes have nothing but good things to say about him. It seems he's the kind of Don Juan that wants girls to be happy even after he's parted ways with them.
At first glance, it doesn't seem like he would have very good compatibility with Shiki at all. The tale of how these two destined lovers met is told in Kagetsu Tohya.
The second Arihiko laid eyes on Shiki, he intuitively knew that he was broken. “I've decided, you're going to be my rival.” So he declared, while eating the pudding he had just stolen from Shiki's lunch.
In the side story “Nanako-chan S.O.S.,” his fate was intertwined with the delinquent elemental Nanako, but normally he hates stuff like ghosts and monsters.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised