Written by Seishi Kishimoto, younger twin brother of Masashi Kishimoto — yes, that Masashi Kishimoto — 666 Satan, retitled "O-Parts Hunter" in the North American release for rather obvious reasons (though otherwise a faithful translation), is about Jio Freed, who, after joining up with a girl named Ruby Crescant, starts searching for artifacts of an ancient civilization, called "O-parts", so he can take over the world.And Jio has Satan sealed inside of him.The series gained a bit of controversy in Japan as many readers accused it of being a blatant ripoff of Naruto. Being the younger twin brother of the author of that series didn't help matters. It got so bad that Masashi had to step in and pretty much tell everyone that wasn't the case and that both brothers just had the same influence (i.e. Dragon Ball) for their works.This was Seishi Kishimoto's first series, followed by Blazer Drive.
Big Bad: Actually averted. Several villains would like to be the Big Bad, but none of them can be held responsible for everything, nor could they have become what they were without the O-Parts, Recipes or Shin.
Although Beelzebub and Satan are the closest to the actual thing.
Bittersweet Ending + Distant Finale: Jio is able to defeat his Satan side and save the world from the Shin ship but he, along with the other Kaballahs (Ruby, Zero, Jack and Cross), teleport it into space with them on it, destination unknown, in a effort to keep it from being misused again. The final page of the comic is a grown up Ball, married to Mei, telling this story to their two children.
Dark Is Evil: Played with. Demonic Recipe Holders (eg. Jio himself) can be decent people. Unfortunately, this power is still evil and threatens everyone around them. Moral alignment doesn't determine Recipe compatibility, it seems...
Nei in Avenger has heterochromia to indicate that she (or he, it ain't exactly clear, either) is both a Robot Girl and a real human. Well, actually, she/he is neither but has the potential to become one of the two.
Eldritch Abomination: In spades: every single demonic or angelic recipe is an Eldritch Abomination of notable horror. The most normal-looking (in comparison to the others, at least) would have to be Naamah, number 10i on the Reverse Kabbalah: it looks like a giganitc whale creature in which the main villains hold their base of operations. Some of them, mostly the angelic recipes, can be classified as Humanoid Abominations as well.
Jio's costume comprises odd shoes (one is a white converse and the other a black school shoe) a black one side 'n' white the other top and hair. This is displayed to be a visual reperentation of his split personalty. Indeed when he goes 666 on enemies' asses his hair turns all white. In the 3rd part there is a scene where a black Jio (good) and white Jio (satan) are having 'words'.
Floating Continent: Roc Bird City, built around the body of the demonic recipe Beelzebub.)
"Freaky Friday" Flip: Astaroth has the power to turn one thing into another, he was only shown using it to switch two people's bodies, and to make them very, very heavy.
Market-Based Title: Viz understandably didn't want to deal with the protests they would have faced had they released this series in English with its original title, so it became O-Parts Hunter in the United States.
Meaningful Name: His name is Ball and he loves spherical things and hates sharp edges. Pretty damn obvious.
Morph Weapon: The O-Parts are notable for this. Even more so after the Time Skip.
Pretty Fly for a White Guy: Ball, to an extent. Combined with the dreadlocks, if one were to avoid looking at any color pages, they could go the entire series and not realize that he's not actually black.
Sealed Evil in a Can: Jio with Satan might look like this, but Jio is actually a personality that was forced off of Satan and had priority control of the body they shared.
Sibling Yin-Yang: Kirin and Kujaku are twins. Kirin inherited the Cyclops third eye from their father, Kujaku inherited the O.P.T abilities of their mother. Kirin is good, Kujaku is evil. Do I have to go on?
Tearjerker: Quite a few here and there but the most heart wrenching being when Jio finds a couple who think of him like a son, and being parentless this is a big deal to him. At one point the get into an argument and Jio run off. He eventually cools down and goes to apologize but the baddies have found them and killed the couple in cold blood.
Time Skip: Two actually: a somewhat subtle one from child to pre-teen and another to teenager.