Exoskull Rei

Kamikaze Zero (looking suspiciously like Kakugo) armors himself to fight off an extremely immoral group of bullies. All Yamaguchi Takayuki fans will love this. Like Cyber Momotarou, it's just one volume but has a continuous story instead of random standalones. I must put a warning on this one though. It's got nudity, violence, gore and a strange sort of sexuality going on throughout the entire thing. It's weird but from this guy, not something you'd be too surprised about.

Kakugo and Harara are two siblings who have been trained to fight demons, monsters, and other creatures that are now roaming about in the post-apocalyptic 21st century. Along with their superior fighting techniques and specialized skills, they were also given special suits called Zero armor, which were made from the souls of dead warriors. Unfortunately Harara succumbs to evil within the Zero armor, and Kakugo is the only human alive that has the skills and equipment to defeat his sibling.