Complete Monster/O-Parts Hunter

File:Python jack.jpg
While *666 Satan* (retitled *O-Parts Hunter* in the United States) is a rather idealistic manga, Jio Freed and his companions have encountered many enemies, ranging from human criminals to angels and demons. Here are the worst of the worst.

* Satan is the most powerful and wickedest demon of the Reverse Kabbalah. Formerly a scientist of Solomon called Lucifer, he helped to create the two Kabbalah to harness the universe's energy, which eventually absorbed his native planet and later caused the destruction of many others. Obsessed with power, Lucifer sacrificed numerous creatures to become Satan, but this caused him to lose his body, so he decided to consume the bodies of the other scientists of Solomon until he was tricked and his consciousness sealed in the body of human child Jio Freed. After landing on another planet, Satan caused several deaths throughout history and contributed to various mass extinctions, notably one provoked by Alucard Spirit. During Jio's childhood, Satan did everything to ensure he'd have a horrible life and corrupted him into hating the rest of the world to feed on his negative emotions. Around this period, he also ruined several people's lives to consume their hatred, killing Jin's parents and later murdering Cross's sister Lily, destroying their town in the process. During Jio's teenage years, Satan often takes control of his body, and usually attacks anyone present. In the city of Rock Bird, Satan takes full possession of Jio's body, absorbs Ruby's soul and tries to destroy the city. Years later, Satan initiates Armageddon to consume all living beings on the planet, but not before corrupting them all into turning against each other to feed on their hatred. He then plans to travel to another planet and become its supreme ruler.
* Amaterasu Miko is the leader of STEA government who intends to assimilate all of the living beings on Earth with herself with the intent to unify all thoughts. To this end, she attempts to activate both the Kabbalah and the Reverse Kabbalah to further her own goals. Responsible for the war between O-Parts users and STEA and the experimentation on innocent people, Miko refers to the people she manipulates as nothing but mere tools for her to use. After STEA's main base was attacked, Miko then bodyjacks several people to claim the legendary O-Part that could activate both the Kabbalah and Reverse Kabbalah. After claiming the legendary O-Part, Miko then initiates an Armageddon which would destroy the planet while assimilating with all of the living forms. Despite her well-intentioned claims, Miko is nothing but a woman with a god complex who views believes everyone on Earth should cater to her whims.
* Balsa is an ambitious and cold-blooded STEA member, and Cross's second-in-command on the Shin. In his first scene, Balsa sacrifices his own soldiers to test his enemies' firepower, and gets increasingly worse in following appearances. While on a mission in Rock Bird City, Balsa activates Shin's most dangerous weapon, Zoru, which consumes the souls of his crew members and countless innocent citizens, in order to destroy the entire city in an attempt to assassinate Cross. Aspiring to overthrow STEA and rule the world, Balsa later uses Shin's destructive powers to becomes the supreme ruler of a new nation, and also starts searching for the angels and demons of the Kabbalah, planning to exploit them to achieve his goals.
* Sabaki, an early villain, is a ruthless Arms Dealer who illegally sells dangerous O-Parts to criminals. Occasionally, he also tricks gullible people by selling them unstable O-Parts, including a mayor whose entire city was destroyed due to his machinations. Introducing himself by killing his servant for a minor reason, Sabaki plans to obtain more powerful O-Parts, so he hires several mercenaries to pillage the Thorn Ruins and assassinate his former colleague Lang who opposes him. Later on, he has said mercenaries be burned alive to test the power of his new henchman. Eventually, Sabaki goes to the Thorn Ruins himself, where he severely injures both Lang and Jio Freed, and tries to kill them in a painful way. Greedy and sadistic, Sabaki proves that even in a world with demons, humans can be the real monsters.
* Jagga is the tyrannical ruler of Entotsu City, who assassinated his predecessor to take his place. In order to excavate the powerful O-Part Mekisis, Jagga had all his citizens forced into slavery for years, executing anyone who tried to escape and enslaving every strangers who came to the city. Jagga also hired criminals of Zenom to constantly terrorize the city, so that the population would become dependent on his protection. After capturing the young teenager Ruby, Jagga has her stripped to her underwear before making perverted advances towards her. When Mekisis is discovered, Jagga takes control of it and uses it to ravage the city, killing countless civilians and soldiers in the process. Believing himself to be a god, Jagga plans to use Mekisis's powers to conquer the entire world, no matter how much deaths and destruction will be necessary.
* Wise Yuri is an underling of Zenom known as the "Crimson Magician" since he murdered hundreds of people in bloody ways. A vicious maniac, Wise even killed some of his own allies, and added new piercings to his tongue for each victims in pride. Instructed to find the O-Part Mekisis, Wise allied himself with the tyrannical Jagga, and made his oppression of Entotsu City even worse by constantly attacking the population with his henchmen. After Mekisis is discovered, Wise sends his henchmen to murder any citizen who could potentially pose problems. Eventually, Wise betrays Jagga to steal Mekisis from him, and uses it to wreak havoc in the city in order to test its powers, planning to use it to Take Over the World. Despite his low rank, Wise is one of the worst Zenom has to offer.
* Museshi is Wise's equally bloodthirsty right-hand-man, who has a penchant for using his O-Part's claws and fangs to torture and slay his victims, even eating them on occasion. He assisted Wise in many of his crimes, including during his mission in Entotsu City where they helped to enforce Jagga's tyrannical rule by constantly attacking the citizens to ensure their subservience. The day Mekisis is discovered, Museshi gleefully participates in the ensuing massacre of civilians ordered by Wise. When he sees another Zenom member threatening the young Ball and his friends, Museshi brutally murders him in order to have the children for himself. He then proceeds to viciously attack the kids, even attempting to devour Ball alive.
* Ikaros, in truth the demon Beelzebub, is the ruler of Rockbird City, which he controls in a nightmarish dystopia of terror and brainwashing. Keeping Rockbird afloat via a ghastly method, Ikaros promotes a famous martial arts tournament with often deadly consequences, only to take the winner and use them as a Living Battery to power his domain until they are used up in four years. Upon discovering Jio and his inner demon Satan, Ikaros wishes to possess the more powerful demon, trying to kill Jio's allies and torture his close friend Ruby to draw Satan out so he may use him to raze the world until it submits to him.
* Python Jack is a sadistic Serial Killer famous for his love of murders. Long ago, he was treated like a monster in his village due to his disfigurement, so he decided to become a monster for real and went on a killing spree. Thereafter, he committed many mass murders just for fun, even destroying a few cities. After joining the Rock Bird Tournament, Python brutally slaughters all the other contestants in his qualification match, and later kills many innocent citizens in horrific ways. During his match against the young girl Yuria, Python tries to make her suffer as much as possible before killing her. Despite his tragic backstory, Python happily embraces his past, as it made him the heartless monster he was meant to be.