Blood+ - City of the Night Walkers is a pre-sequel to the animated Blood+
[This is a spin-off of the anime Blood +, but focuses on Kagi only.] In Hong Kong, Nishi is an young policeman investigating a case of strange murders, where the victims appear to have died of a fatal blood loss. But the investigation is taken over by a mysterious organization, whose member warns Nishi that he had better not keep on trying to uncover the truth.
A while after, as Nishi wanders in the city, he is suddently attacked by a monster, but hopefully a (beautiful ^^) stranger saves him.
This manga is really interesting and wonderfully drawn ; from what I've read, the plot seems very deep and there's still a lot of mysteries to unfold...