From MangaHelpers:

One of the most infamous criminals of this country is literally a monster. Vampire Zeo is immortal and invincible. He takes whatever he wants from whomever he wants whether it's from a company or even the government. And when the police or military or even seasoned assassins that specialize in monster killing go after Zeo, they all end up running away with their tails between their legs.

So when a young girl named Lin tells Zeo that she has come to make Zeo atone for his crimes, he laughs it off. However, Lin is from the Church, and, although she is weak, she uses a special spell that combines their souls together, making them essentially one person. Whatever pain she feels, he feels and if she dies, he dies too and vice versa. However immortal he may be, her death would mean his own demise and he has to protect her from those who want Zeo’s head on a platter. Her original mission from the Church was to make him stop his criminal activities, but when Lin learns that the Church wants him dead, she questions her orders and joins forces with Zeo until she learns the truth. [vyc]