Shido is not your average Private Eye. A human at heart but vampire by trade, he's a half-breed living and working in the seediest parts of Yokohama. While we don't see the exchange of monetary notes between client and professional, we assume his main cash-cow comes from the consistent run ins he has with Nightcrawlers - nasty creatures from the other side of life who are causing all sorts of havoc around town.
A Nightcrawler's main ambition is to take over a living or once-living vessel and turn it into a rabid cannibal as quickly as possible. It's style ranges from causing accidents and possessing corpses (zombie action), possessing the sick who turn rabid (rabid zombie action) and tricking star-struck fools to willingly let them take over in a more passive way and grant them fame in exchange for some cannibalism during down-time (Jeckle/Hyde action). No matter what the method the results aren't great for the average punter.
When the possessed turn it up, everyone cops it sweet. When they kill a dog they rip it in half and eat the intestines. When they cut off someone's head they take the shoulders with them for good measure. The gore depicted throughout Nightwalker is in a word, brutal.
As with any classic vampire story, lust has a fairly prominent place in Nightwalker. There is plenty playful humour at the start of the series between what Shido's cohort Yayoi is saying, and what the cute, young protégé Riho is hearing ("He'll have WHAT for dinner???"). As the story develops though the inevitable classic combo of sex & violence plays a more central role, and the nudity becomes more and more prevalent.
With an electric organ soundtrack straight out of Henry: Portrait of a Serial Killer, along with some clunky animation by today's standards, Nightwalker has some dated elements to it. But the character designs are strong, and the story by Ryota Yamaguchi (Cowboy Bebop, One-Piece) takes a page from nearly all the classic European horror tales and develops nicely, as Shido has more and more tough decisions to make that impact those close to him ("do I let her die, or bring her into my own hell?")