Blue Soul (青い霊魂 / Aoi Reikon) is a fan series published by OniTsuuga. The main characters are Rin Okumura, his twin brother Yukio and his sister Aoi Okumura, the little sister whom the brothers had no knowledge of since she grew up in Gehenna.
Taking place only days after Rin and Yukio defeated Satan during the Great Armageddon - the night in which Satan summoned the Gehenna gate to merge Assiah and Gehenna, Mephisto was re-established as the Chairman of the True Cross Order, yet allowed Yukio to remain Paladin for his efforts in that night and Shura Kirigakure was also appointed Paladin. Everything in Assiah returned to normal, or so one might think..
Not long after Satan's defeat, a series of deaths occur in the Academy, numerous students die mysteriously of blood loss; No blood is found in their bodies. Shura was made Chief Investigator in that matter, trying to catch the murderers before they have a chance to murder more students. One night, the Exwires notice Shiemi is missing. Rin, much to Shura's displeasure, insists on searching for Shiemi as he theorizes that she might fall victim to the killers. Yukio however agrees with Rin and they both search for Shiemi.
They find her in the school cathedral, chained up with her clothes ravished. The mysterious killer, a female, makes an appearance and immediately recognizes Rin's blue flames; she erupts in maniacal laughter and proceeds to fight Rin and Yukio. However, as it's revealed they stand no chance whatsoever as that foe is simply too fast and fatal. Before she can kill Rin, she's struck by a blue ray of flames, not coming from Yukio.
A mysterious girl appears, her right eye illuminated by a blue flame. The female foe rages and attacks her, but the girl blocks all her attacks. After a long battle she kills the foe with her sword, also illuminated by blue flames. Not long after the foe's defeat, Shura arrives at the cathedral, not very pleased by the fact that Rin and Yukio have been defeated. Mephisto grinned when he looked at the girl and took her with himself and Shura to the principal room. Rin and Yukio were given no explanation.
The next day she shows up as a student of the cram school and reveals herself to be Aoi Okumura, Rin's and Yukio's younger sister, born to Satan and - that's how she calls her - Queen Yuri Egin, her mother. Yukio also informed that he, Rin and Aoi were to become a special team for hunting those killers, whom Aoi reveals to be vampires. A story about three children of Satan who have to go fight multiple foes..
Rin Okumura (奥村 燐 / Okumura Rin)
The main of the mains. Son of Satan, King of Gehenna, and Yuri Egin, Queen of Gehenna (according to Aoi). In a nutshell, he's a son of Satan from a human mother. His powers were sealed by Mephisto Pheles inside the Demon Blade Kurikara, allowing him to lead a human life for 15 years. He lived with his foster father Shirou Fujimoto until he was possessed by Satan and thus committed suicide to prevent him from dragging Rin into Gehenna. After Shirou's death, Rin faces Mephisto who wants to execute him at first, but after Rin showed his determination of defeating Satan, Mephisto takes Rin under his wing and introduces him as a student in the True Cross Academy as a new student of the Cram school. Rin decides to become a Knight (a swordsman exorcist) and acquired a demon cat familiar called Kuro, who was previously Shirou's.
Yukio Okumura (奥村 雪男 / Okumura Yukio)
Rin's younger twin brother and also Satan's and Yuri's son. Unlike his brother, he didn't inherit his demon powers at birth since he was too weak at that time. He secretly trained to become an exorcist during his childhood (through Shirou) in order to protect Rin. Although he was a first-year student at the time Rin entered cram school, he was already a professor at the cram school and teaches anti-demon pharmalogy. In the advent of Ernst Egin (Yuri's father) becoming pope, Yukio became Paladin and was allowed to keep that title even after Egin's death. After the Great Armageddon, Yukio abandons his gun and seems to become a homage to the old Shinobi, wielding Kunai, Shuriken and medieval-style daggers as Dragoon weapons. And boy.. he has many of them.
Aoi Okumura (奥村 青い / Okumura Aoi)
Rin's and Yukio's younger sister and - unlike her brothers - a Demon Princess of Gehenna. That's why she's often styled Your Highness (jap.: 殿下 denka) or referred to as hime-sama or Aoi-hime by demons, and surprisingly even by Mephisto himself, despite her being his half-sister. Anyways.. she's a 13 year old girl who grew up in Gehenna and seems to hunt vampires for a living. Why a princess of Gehenna would be sent to Assiah to assist the exorcist with the vampire hunt is unknown - currently. She didn't have to go through the Exwire exam, since her defeating that vampire was "the equivalent of an Exwire exam", as stated by Mephisto. The humans obviously don't like her since she's a child of Satan, but Aoi doesn't seem to care. Like Rin, she uses a katana, however it slightly differs from Rin's Kurikara; Aoi's katana is called Muramasa, an ancient Japanese samurai sword which was demon-possessed. Because of that affinity, Muramasa holds the power of devouring the souls of everything it kills, thus making it a powerful weapon. It also proves effective in killing vampires, who are supposed to be immortal. Although she seems to have significant control over her flames, she's also very weak in many things: she doesn't have physical power, can't carry heavy things and is very sensitive to pain. She can't even use demon spells due to her being so young.
Satan is the King of Gehenna. He is said to be the creator of every demon in existance, but no body in Assiah, save his wife Yuri Egin, is strong enough to be his vessel. Not even Shirou Fujimoto, the most powerful Paladin lasted long when Satan possessed him. He murdered a great number of exorcists during the Blue Night, which in reality didn't occur because of Satan's "bloodlust" (he doesn't have any), but because he was trying to find a vessel to save Yuri from being burned a witch. Little is known about him because he appears to be a maniac to everyone he ever meets, save Yuri. He is the origin of the blue flames; Aoi says Satan was created from fire by God. Fact is, his name alone instills fear in Assiah.
Yuri Egin (ユリ・エギン / Yuri Egin)
Satan's wife and thus the Queen of Gehenna. She's the mother of Rin, Yukio, Aoi and Leviathan and is said to be a very gentle and kind person. Upon her death after she gave birth to Rin and Yukio, she entered Gehenna since she was possessed by Satan before death. Satan proclaimed her his consort namely the Queen of Gehenna and all demons. Going by what Satan showed Yukio, Yuri determined that demons weren't inheritly evil but rather curious. She never had any bond with the Vatican at all, unlike her father who was a very popular cardinal. Because of her mating with Satan and carrying his children, Yuri was sentenced to death on the stakes by her own father. However she was saved by Satan albeit the horror of the Blue Night was unspeakable even for her. Aoi adds that she's "a woman of great strength and a real role model", which implies that her power has grown ever since her death.
Mephisto Pheles (メフィスト・フェレス / Mefisuto Feresu)
Satan's eldest son. He uses the name Johann Faust V. in public, which is a homage to the old German Faust legend. Heck, his very name - Mephisto Pheles - is taken from the Faust saga, which Mephisto claims has really come to pass. Well.. he's the principal of the True Cross Academy, a Honorary Knight of the True Cross Order and also a demon prince of Gehenna.. not much to say about him, except that he keeps everyone guessing who's side he's on.
Amaimon (アマイモン / Amaimon)
Also Satan's son and Mephisto's younger brother - thus his second-eldest son. He's called the Earth King because all earth demons are his dominion. He has a sweets fetish and a love for his familiar, Behemoth. He also is interested in Shiemi Moriyama, although their relationship is quite unclear as of now. He likes to challenge Rin every now and then.. under orders of Mephisto of course. He's one of the most powerful demons ever.
Leviathan ( リバイアタン / Ribiatan)
Yuri's and Satan's youngest son and only 8 years old. He's very noisy and refers to himself as The Awesome Me (jap.: おれさま / oresama). But he's just a kid. He has no abilities whatsoever (except his flames which are eyecandy as of now) and annoys the hell out of Rin with his pompous self. Always with Leviathan is his friend (or demon maidservant?) called Zelda who's also 8 years old. And also, because his name Leviathan is too long, people refer to him simply as Levi.
Shiemi Moriyama (杜山 しえみ / Moriyama Shiemi)
Rin's first friend ever since he joined the Academy. She's shy, chaste, loves nature, thus having a powerful familiar whom she calles Ni-chan. Her relationship with Amaimon seems to be quite unclear although she's fond of him. And like Amaimon, Shiemi also loves sweets.
Shura Kirigakure (霧隠 シュラ / Kirigakure Shura)
The newly appointed Paladin and Rin's mentor. She's sarcastic, beasty and really powerful. Like Rin, Shura isn't human but a demon cat. She was saved by Shirou from the path of evil though. She appears to have an affection for Mephisto, although that isn't quite clear, similarly to Shiemi and Amaimon. She gained the Meister title of both Knight and Doctor. Her sword specializes on the mist and emits snakes.
Hogosuru Kirigakure (霧隠ホゴスル / Kirigakure Hogosuru)
Shura's elder sister and also a demon cat. Not much is known about her except that she disappeared years ago and never came back.
- Rin's name translates into phosphorus, the Latin equivalent to the word lucifer, which the name used for Satan when he was cast out of heaven according to the Bible. Both words mean light bearer.
- Yuri Egin is a homage to two things: The satanistic prayer "Satan, Yuri and You" and the demon Lilith, Satan's consort in the Jewish Talmud.
- In the German legend Faust, there was a demon called Mephistopheles who tempted the main character Johann Faust.
- Amaimon is derived from the ancient demon Amaymon, who's supposed to be one of the Princes of Hell according to demonology.
- Leviathan is one of the seven princes of hell, representing the sin of Envy. It's also the title of one of Thomas Hobbes' literature works called Leviathan.
- Muramasa was a Japanese swordsmith in the 15th century and the sword Muramasa was claimed to be demon-possessed by Tokugawa Ieyasu, a shogun.