Rokudou Tousouki

En has no prospects as a child of the slums—he ekes out a living by selling the scrap cyborg augmentations he scavenges to anyone who will buy. But when he's forced to go through some unethical experimentation, he transforms—making him eligible to become a fighter in the only thing that matters in this bleak world: The Rokudo Tournament!
(Source: Kodansha USA)
Note: Includes 1 extra chapter.