Jio is a young boy interested in achieving one thing: to conquer the world. He meets the treasure hunter Ruby Crescent and soon learns about the amazing weapons O-Parts and those who are able to use these weapons O.P.T.S. Jio ‘s dark and lonely past lead him to trust nothing in the world except for money and himself however he later finds himself travelling with Ruby (rather reluctantly) as her bodyguard. After a short time together, Jio and Ruby part ways (as Ruby runs out of money to pay Jio for being a bodyguard) but almost immediantly Ruby is kidnapped by the man with 666 on his forehead, Satan. Jio attempts to rescue Ruby, realizing that he himself is a O.P.T.S. but is beaten quickly by a strong O-Part. Immediantly after this, the numbers 666 appear on Jio’s forehead, revealing that Jio is the real devil Satan and instantly defeats the fake Satan. Jio decides to continue travelling with Ruby until she can pay him for being a bodyguard.
666 Satan is the story of a young soon-to-be world conqueror, Jio Freed and the up-start treasure hunter, Ruby Cresent. It takes place in a fantasy world in which there are magical items called “O-parts”. O-parts have magical efects ranging from fire to doubling strength. The only catch is that only special people called “O.P.T.s” can use them. Jio must soon give up his withdrawn nature to save Ruby. In the battle to save Ruby, Ruby find out he has a demon inside of him, which Jio is clueless about. This demon is the legendary demon-Satan and on Jio’s forehead, as a sign of this demon appears the “666″-sign of the beast, which recedes leaving Jio with no memories of what happened. This starts an adventure of a lifetime were Jio learns more about himself, his friends and the demon inside while fighting against the Stea Government and the evil Zenom Organization. He gains friends along his journey who he learns to care for.