Despite this manga's title, the manga is not about Satan/devil worship of any kind. While Satan is a character present in the story, he is portrayed as a powerful demon beast sealed inside the main character, Jio Freed.
The basic plot:
Similarities to Naruto (for those of you who may be skeptical about whether this manga is good or not):Wikipedia wrote:Ruby Crescent is an ordinary girl, who likes to go out with boys and go shopping. Her life is changed dramatically when her father dies and she becomes a treasure hunter as he was. Her objective is to find O-Parts: magical items hidden under the ground which grant people superhuman powers and can only be used by an O.P.T. (O-Part Tactician), Angel, or Devil. She soon meets a mysterious boy named Jio who, due to having a dark, lonely past, seeks to conquer the world. Jio is hostile to her at first, but ends up traveling with Ruby as her bodyguard. When Ruby is attacked by an O.P.T. claiming to be Satan, Jio rushes to her rescue and a battle occurs. Initially they are on the losing side, but Jio releases his true power and is revealed to be not only an O.P.T., but the real Satan. Thus, the two continue to travel together in hopes of unlocking their pasts.
666 Satan's battle system doesn't utilize chakra. Instead they have spirit energy and fight with o-parts.Wikipedia wrote:This manga and its characters have similar traits to the manga series Naruto, mainly because the creator is Seishi's twin brother, Masashi. When 666 Satan was released in Japan it was accused of being a copycat to Naruto[2][3][4][5][6][7]. This can be explained by the fact that the two are brothers. In turn, both share similar influences and similar artistic styles. Both manga were deeply influenced by Akira Toriyama's Dragon Ball and have incorporated many elements from it into their own stories.
Examples of similarities between 666 Satan and Naruto include, but are not limited to:
* The main character of both stories lack parents; and substitute others for parental and familial figures. Naruto shares this trait.
* Both protagonists in each manga (Jio Freed and Naruto Uzumaki respectively) have a very powerful demon inside of them (both the most powerful of a series of demons in each series), such as Naruto's Nine-Tailed Fox and Jio's Satan.
o Jio and Naruto both share certain physical characteristics of the demons they each play host to such as Naruto's whiskers, eyes and canine teeth and Jio's half-white hair and one red eye.
* Hard childhood is important for the storyline of both Naruto's and 666 Satan's protagonists. Both Jio and Naruto start friendless because of the demons inside of them which cause them to be shunned by society in general. Both come to the "conclusion" that they need to become the strongest person around, in order to conquer the world/ become hokage.
* Both Naruto and Jio were trained by hermits, Jio by Kirin and Naruto by Jiraiya. Jiraiya and Kirin look somewhat alike, with long silver hair, and both are very strong.
* Both Naruto and Jio cannot control their demon.
o Both have had outside influence in learning how to suppress their demons (in a literal sense).
+ Both stories also have organizations trying to capture these demons.
+ Both Jio Freed and Naruto Uzumaki have "talked" with their demon's and in some cases, even had altercations with them.
+ In certain instances, both Jio and Naruto have had to make usage of their demon's power.
+ Kirin (666 Satan) and Kakashi (Naruto) have each lost their left eye and are in possesion of a unique eye (The Sharingan Eye and the Cyclops Eye respectively)
By all means, every Naruto fan here would do themselves a favor by checking out this manga. If you love watching/reading Naruto then do be sure to give this series a try. I'm positive everyone here will have fun reading this.
For more information consult Wikipedia: http://en.wikipedia.org/wiki/666_Satan