Apocalypse Zero

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.

Synopsis from Anime News Network