Byakuda no Hanamuko

From Akita Shoten:

A white snake who is a divine messenger and a childhood friend with a mystery. Entangled obsessions in a Japanese-style fantasy. Tsubaki, a high school student with a predisposition for bad luck, has a one-sided love for his childhood friend Sakuya, who is both beautiful and talented. Then, one day while leaving school they are randomly attacked on the street. In the ancient capital Kyoto, a lovestory-defying fate unfolds.