Clamp Premium Collection Tokyo Babylon, Vol. 1 (manga)

Subaru Sumeragi is the thirteenth head of his clan and a powerful onmyouji. With the help of his twin sister Hokuto and the veterinarian Seishiro, he solves supernatural troubles in Tokyo and helps whomever he can. However, not all onmyouji are so kind-there are some who use their power to kill, and unbeknownst to Subaru, he has a history with one such assassin...