Battle Of The Six Realms summary:

In a world where humanity escaped underground to survive, En is an impoverished boy who lives off the trash of those more privileged than him. He spends his days in the slums, looking through a trash heap for the decomposing bodies of "losers," those who were defeated and killed in the "Battle of the Six Realms." To participate combatants have to undergo an expensive "body modification surgery," and En sells these modifications, machines attached to their bodies that significantly increase fighting ability. Uninterested in the tournament and its champion, unlike everyone else, he aims only for a worry-free life. But one day, a chance encounter with a mysterious man shatters his reality and puts him on a different, more gruesome path.