After causing the death of his parents, a young boy lives solely for the sake of his sister - avoiding contact with other people for fear of hurting them like he did his family. But one day, a mysterious stranger appears and says "I will control your life from today onwards"...

From MangaHelpers:

In response to a decrease in population, human corpses have become a major resource for society's labor pool. Necromancers are needed to animate corpses for various jobs. These corpses work reliably unless they encounter human love, at which point they go berserk and attack. Zen is the necromancer of his city, and he spends his time animating corpses and dealing with the messy aftermath of those cases of 'necrophilia.'

Zen has a strange assistant, who keeps his face covered and never speaks in public. This assistant is Yomai, a ghoul that Zen took under his wing. Zen allows Yomai to feed from his own flesh, but he has strictly forbidden the ghoul from revealing himself to, speaking to, or eating other humans. Yomai doesn't want to anger his benefactor, but he is not completely satisfied living under all of Zen's restrictions. There's plenty of violence, death, and surprises ahead for the necromancer and his ghoul! [tethysdust]