Mind, body, soul. A treatment by Stewart Wilson

Focus in on Mark Hammond, a pulp hero for the new century. He doesn't wear a trenchcoat, doesn't dress all in black. He's just a man off the street.

Body
Blue jeans, boots, sloganed T-shirt. Black leather jacket and fedora (these never change). Not heavyset, but just well built enough. Two days' stubble on his chin. Dark hair, cropped close. Piercing green eyes. On a microcellular level, he is different from everyone else. His cells have bonded with microscopic machines, making him stronger, faster, and sharper than anyone has a right to be. They also help him heal, which he tends to need a lot.

Mind
The nanotech acts as a second brain, leaving him in two minds about everything. Originally, Mark resented feeling like a watcher in his own brain, now he's come to work with the second consciousness as a team. Mark is streetwise, savvy, though not overly sharp. This other mind, the Scientist is a scientific genius, but seems to be lacking in ethics and scruples on occasion.

Soul
The Scientist was dead for a while. His mind downloaded into a set of inert liquid technology. He knows what happens after death, and when he was reawakened by Mark, his soul came back. There are others with two souls, but theirs have been cast out of Hell or worse, raping victims by supplanting themselves into other people's bodies. The Scientist can sense this, and wants it stopped.

The power of Three
Everything in these stories happens in threes. The old Wiccan belief in threefold return (what you do comes back upon you threefold) most certainly applies, but not in ways you'd think. Make the threes obvious (Mark's gun, for instance, chambers three rounds, which are each engraved with a triangle), but don't let the story suffer for their
presence.

Spoilers
The first couple of stories should be straight up "find the possessed guy and whack him" in outline. The second one draw parallels with Mark's previous life. The third one we learn that the cops are hunting him down, and he needs all of his boosted abilities to escape from a couple of possessed policemen. [Anything from here on in isn't fixed] The fourth, start with the shooting of a possessed old woman. Against the Scientist's wishes, Mark goes in search of evidence of her being possessed, and finds none. Cue big argument, and the Scientist taking over Mark's body to make a point. The fifth is where it all comes out. None of those people are possessed at all. There are no other souls, and before being incarcerated in the nanotech prison, the Scientist was a serial killer. He's been using Mark to carry out a bloody swathe of murders. In a fit of rage, Mark takes his own life rather than
continuing.