Exoskull Rei

At the end of the 21st century, humans have made great advancements in technology and were able to create a giant energy source to power that technology. However, this new source of energy overloaded and turned into a more devastating force than nuclear weapons. The catastrophe wiped out the majority of human civilization and those who survived lived in terror of the anarchy that took over.
Hagakure Kakugo comes from a secret family that has protected people for generations from powerful organizations and evil from taking over. They are only awoken in time of need and it has been decades since Kakugo was conscious. His body had been heavily modified by fusing it with metals to enhance both his offense and defense. He uses an armor called the Exoskull that attaches to him to greatly boost his already superhuman power. Thus begins a story about a young man and his armor's journey through hell.
(Source: MangaHelpers)