Inui Arihiko is the best friend of Shiki.
Inui Arihiko is a side character from Tsukihime and Kagetsu Tohya. He is Tohno Shiki's classmate and best friend. When Shiki was living with the Arimas, he often used to hang out with Arihiko. He has a competitive attitude against Shiki; Arihiko wants to get a girlfriend before Shiki does. As the player moves through the story, the player will discover that sometimes, Shiki cannot grasp how he became friends with Arihiko in the first place, since Shiki believes Arihiko has a "weird personality." Yet at one point in the game, when Shiki collapses in class due to his condition, Arihiko is the one who loudly informs the teacher that Shiki is sick and needs to be excused. Arihiko then proceeds to help a physically-weakened Shiki to the medical office, thus demonstrating a personality trait that might, depending on the player's subjective perception, serve to explain why Shiki continues to be friends with Arihiko despite Arihiko's sometimes-callous behavior.
As explained in the Kagetsu Tohya side story "S.O.S. Nanako-chan", Arihiko's parents died when he was young, and he lives with his sister, Ichiko. In that story, he stumbles upon and eventually befriends the Seventh Holy Scripture's spirit, and gives her the name "Nanako".
Arihiko plays an important rule in Shiki's school life; that means he primarily appears in the beginning of Tsukihime (providing a lot of exposition) and the school parts of Kagetsu Tohya. He does not, however, appear in any of the endings of Tsukihime, and Hisui's and Kohaku's route barely feature him at all.