The O-Parts Hunter manga series, known as 666 Satan in Japan, features a cast of characters created by Seishi Kishimoto. Set in the near future, it tells the story of a teenage girl named Ruby Crescent who wants to become a treasure hunter, following in the footsteps of her father. Her objective is to find O-Parts, ancient relics that can only be used by O.P.T.s (O-Part Tacticians), people who are able to release their Spirit and focus it into an O-Part to activate a special Effect, usually of a fantastic nature. She meets a mysterious boy named Jio Freed 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.

Throughout the world, there are ten Angels and ten Demons with amazing powers, some of which are sealed within humans. They are being searched for by the Stea Government (Angels) and the Zenom Syndicate (Demons) who want to insert them within the Kabbalah and Reverse Kabbalah, that are found on the North and South Pole respectively. A legend tells that when either Kabbalah is filled, a super weapon called the Legendary O-Part will be awoken.

Protagonists Jio Freed is the thirteen-year-old protagonist with half-black, half-white hair, a very long red scarf and dreams of world domination. He has great battle skill, an obsession with money and is not quick to show his feelings, often hiding them or blaming other factors. He meets Ruby, a young treasure hunter, and agrees to become her bodyguard. Jio started his life without friends, being picked on and beaten by others. The people in his village forced him to leave after the death of his only friend's, Jin, parents for which he was blamed. He was trained in the wild by a wolf named Zero for a year, and draws fangs on his cheeks in tribute to his master, before being chased away to complete his dream of world domination. After gaining Ruby and Ball as friends, and training under Kirin, Jio becomes more trustful of people and slowly breaks away from his dream of world domination, choosing to be more helpful and caring of others. This new outlook quickly gains more friends such as Mr. Wick and Mrs. Vercil, who he sees as his father and mother, never having known his real parents; Jio is emotionally crushed when they are killed by Kujaku in cold blood.

His main O-Part is a boomerang called (C rank) which doubles the energy it receives. He named it after his master Zero. After being repaired by Kirin, it is upgraded to the three-winged (B rank), which triples the power. Jio is the container of Demon No. 1i, , and seems unaware of Satan's existence, not knowing why the people in the village hated him. But during Jio's second fight with Wise Yury, Satan informs Jio that his body, "the vessel", is ready and grants him the . Appearing on the palm of Jio's left hand and reading the number 666, it is an O-Part with the Effect of Energy Absorption. While the mark allows Jio to absorb and use another O.P.T. or Demon's ability, it also gives more and more control of his body to Satan.

During their fight on Rock Bird, Satan and Jio take a fatal wound from Cross and Metatron. In order to survive, Satan absorbs Ruby's soul into Jio. Ruby stays inside of Jio to protect him against Satan's control, and he is separated from his friends during the fall of Rock Bird. Four years later, Jio has become the leader of the Cyclopian village and his O-Part has again been updated, this time to , another boomerang-type weapon that reacts to the amount of hatred in one's heart and changes shape and size accordingly. Jio is discovered by Ball and rejoins his group to rescue Ruby from Stea Government HQ. After reuniting with Ruby, the Orphan crew then head for Zenom Syndicate HQ where Jio ultimately defeats Amaterasu Miko, who has initiated Armageddon. It is then revealed that all of the Angels and Demons, and Shin came from a planet called Eden, where 72 scientists tried to harness all the energy of the universe. To accomplish this task, they created the two Kabballahs and Shin, which have the power to absorb energy in the form of information. 20 scientists were in charge of categories of information and eventually evolved into the 20 Angels and Demons based on their category. The scientist that became Satan desired all of the energy for himself and began absorbing everything, but researchers Adam and Eve foresaw this and created a different device called Freedom. When he entered this program, Satan became trapped inside it; Jio Freed. Jio finally defeats the Satan inside him and gains control of Shin, which he warps to an uninhabited planet.

Ruby Crescent is a treasure hunter who is researching the Legendary O-Part like her father, Zect Crescent. After bumping into Jio, she hires him as her bodyguard and they begin traveling together. Despite being only fifteen, Ruby has extensive knowledge of ancient languages. She smiles almost constantly, saying that after her father died, she realized that crying and running away will not solve anything. She always worries about Jio and Ball, especially when they are fighting against other O.P.T.s. Ruby shares a strong friendship with the two boys, but seems to have more intimate feelings for Jio. She carries a Jade Pendant necklace that her father gave her, which is revealed to be the S rank O-Part with over 100 Effects. Proximity to this Key of Solomon is what awakens an Angel in its host.

It is revealed that Ruby is Angel No. 10, , who Zect stole along with the Key of Solomon while leaving the Stea Government. Her soul is absorbed into Jio by Satan during Jio's fight with Cross, in order to save him from a fatal wound. Four years later, Jio and his friends fight their way into Stea main base, where Jio is able to free Ruby's soul and return it to her body. She is now able to use her Angel powers, which include the ability to shrink and grow any object, and alter their design to some degree. At Zenom HQ, Ruby is finally reunited with her father, who claims to have no daughter. But when Shin begins collecting all of the Demons and Angels, Ruby takes the attack for Zect and he regains memory of his daughter. Both are absorbed by Shin and inserted into the Kaballah. But after Jio stops Armageddon, all three are seen with Cross and Zero before warping away from Earth on Shin.

Ball is a thirteen-year-old boy with dreadlocks living in the Stea Government-oppressed Entotsu City with his ill parents and younger sister. A member of the city's resistance movement, he pretends to be an O.P.T. and has a love of round things. He is surprised when Kirin reveals to him that he actually is one and gives him the O-Part (C rank). Its Effect, Magnetism, allows him to repel and attract the stone at high speeds. Cool Ball later gains the ability to magnetize anything it touches, metal or not. Like Jio, after training with Kirin Ball's reflexes, strength, and awareness were raised greatly. Ball has displayed a natural talent of handling his O-Part and displays amazing cunning and strategy, using his O-Part's magnetism effect to fool his opponents, often coming up with plans in the middle of battle. After helping save his city, he joins Jio and Ruby on their journey. Ball is often the pilot of the main characters' transportation vehicle, the O-Part called the . After the four year timeskip, he wields a new O-Part, called (B rank), which are essentially four smaller versions of Cool Ball that resemble marbles. During their attack on Stea HQ, he and May end up fighting her older brother Tsubame, who afterwards, asks Ball to take care of May. Years after Armageddon was stopped, Ball and May have a son and a daughter that they named Jio and Ruby.

Cross Biancina is the fourteen-year-old, religious commander-in-chief of the Stea Government's SS rank O-Part airship, Shin. He uses the government and Shin to frequent the frontlines of battle in order to increase his odds of finding and killing Satan. Before the series began, Cross lived alone with his younger sister, , after their parents left. One day, Jio came to their village and Satan inside Jio decides to instill loneliness, hatred and vengeance in Cross by destroying the village and Lily in front of him. Cross was recovered by the Stea Government as the only survivor and found out to be an O.P.T., later becoming the commander of Shin and beginning his search for Satan. His O-Part is , which consists of five rings each with their own Effect; thumb earth, index electric water, middle wind, ring finger force field, pinkie bomb. Despite them only being C rank, Cross's skill and mastery of them allows him to easily defeat enemies with higher ranked O-Parts. Cross is revealed to be Angel No. 1, , while Lily was a clone created by the Stea Government from the Key of Solomon and Angel No. 10, Sandalphon. He awakens as an Angel during his fight with Jio on Rock Bird, thanks to the close proximity to Ruby's Key of Solomon.

After the four year timeskip, Cross has left the Stea Government and is traveling with the rest of the Orphan Crew helping them look for Jio. Later, he reunites with Ponzu who has purposely become the Demon Lilith, in order to become closer to Cross, whom she has feelings for. He is absorbed by Shin and inserted into the Kaballah. But after Jio stops Armageddon, Cross, Jio, Ruby, Zect and Zero are seen warping away from Earth on Shin.

Kirin is a white-haired, pickle-loving hermit who is easily bored and lives on the outskirts of Entotsu City. He is a licensed O-Part appraiser famed for his knowledge and repair of O-Parts, as well as his genius as a swordsman with his sword . Despite his genius, Kirin is not an O.P.T., but his abilities exceed those of most O.P.T.s. A former Stea Government officer at the same time as Amidaba and Zect, he is now a hermit accompanied by his dog Jaja-maru. Jio and Ball seek him out to fix Jio's O-Part, but in exchange they have to keep him entertained playing games, which also act as training to be O.P.T.s. Kirin meets up with Jio, Ruby, and Ball again at the Dastom Ruins, where they encounter his twin brother Kujaku, before continuing to travel as a quartet. After the four year timeskip, he is traveling with the rest of the Orphan Crew looking for Jio. During their attack on Stea HQ, Kirin and Amidaba fight and kill Dofwa Longinus.

Kirin and Kujaku are the twin sons of a Cyclopian father and an O.P.T. mother, and the nephews of May's grandfather. While Kirin is special in that his Cyclopian third eye was open when he was born, very rare, if not completely unheard of, Kujaku does not a have a third eye, but is an O.P.T. At one point, Kirin attempted to stab his third eye in order to be normal and stop his jealous brother's hate toward him and his father, but the third eye can not be destroyed. So he cut his left eye out, blinding it, and leaving him with two working eyes: his right eye and his Cyclopian one. Inside the Zenom base Kirin fights and defeats Kujaku with the help of May. After the battle Kirin gives Kujaku their father's sword, revealing that their father had intended for him to inherit the sword the whole time.

Amidaba , nicknamed , is a skilled O.P.T. able to use any O-Part. She was a Stea Government army officer alongside Zect Crescent and the one who found Ruby's Jade Pendant. Amidaba meets Ruby when they are both locked up in the dungeons of Entotsu City and helps her escape. She reappears at Rock Bird investigating Olympia and Ikaros. There she uses Mars' O-Part, , which materializes anything drawn in it. After the four year timeskip, she is traveling with the rest of the Orphan Crew looking for Jio. During their attack on Stea HQ, she and Kirin fight and kill Dofwa. Her own O-Part is , seven tattoos on her palms, which each have different Effects, her favorite being gravity control.

Jin