Hyakki Maruo is a supporting character in the anime that once was a well-known professor at Teito University. Due to an unfortunate car accident, Hyakki had all four limbs amputated. Since he knows a lot about prosthetics, Hyakki tries to build ones to replace his older model so he can continue his job as a distinguished surgeon. He is friends with Kuro, in which he operated on him along with Dr. Honma.