The story begins with Soko Kiryu moving to a new area and transferring to a new school, suffering from fainting and dizzy spells due to her frail nature. In the new school, she immediately catches the eye of Kashii, a brute who wants Soko to become his new girlfriend. When she refuses, he tries to rape her in the School Infirmary, but he ends up disappearing and leaving only his clothes behind.
Worried over his disappearance, Soko returns to school and learns that a new guy has appeared and is taking over Kashii's name and position. Nobody remembers the "real" Kashii and this new guy seems just as intent on getting his hands on Soko, though he says it's to kill her, not rape her. He reveals himself to actually be called Akira, of the Byakko family, and that he is tasked with the job of killing Soko, for she is the reincarnation of Soryu, the leader of the bloodthirsty, murderous Oni tribe.
Unbelieving, Soko tries to prove to him that she is not, but when evidence stacks up that she is Soryu, she must admit that it seems to be true. And so they set off on a long journey in an attempt to find out more about Soryu, the Oni tribe and ways on how to prevent them from resurrecting, to take Soko's power of Soryu away from her to turn her into a normal girl.
The manga's background is based on the tale of the Four Gods of Seiryuu, Suzaku, Byakko and Genbu, similar to Fushigi Yuugi.
Ao No Fuuin Contains Examples Of:
- Asshole Victim: Kashii during the first Chapter. He was portrayed as a loud brute and tried to rape Soko, so you don't really care that he was consumed.
- Attempted Rape: Soko is the victim of this multiple times. Kashii tries it, later Akira attempts this and once again by Akira, when Soko refuses to take the intimate way out to get rid of her powers. He doesn't take it well and reminds her that he can easily kill her.
- Big Bad: Takao, being the one who is mostly in the way to prevent Soko from losing her Soryu powers and prolonging the events of the story.
- Brainwashed and Crazy: Soko more goes the brainwashed and lethargic way when Takao keeps her under mind control, bringing about the resurrection of Oni. She does attack Akira when he tries to save her, before snapping out of it.
- BrotherSister Incest: They are merely half-blood siblings, but Mayumi is told to sleep with Akira and have his child, continuing to secure the Byakko family and its pureness.
- Character Development: Particularly noticeable in Soko, who begins as a fearful girl that doesn't want to be Soryu, but eventually (though unwillingly) decides to step up her game and take her position as the ruler of the Oni.
- Chekhov's Volcano: Mount Fuji is mentioned as one of the potential areas to be the location of the Blue Sanctuary, but in the final chapters, Fuji is presented as a threat as it will most likely erupt within 24 Hours.
- Child of Two Worlds: Soko is portrayed as this and it brings her a great deal of insecurity and torn loyalty as the story progresses. She is the Queen of the Oni, but more identifies herself as human and wants to be just that. But the Oni depend on her as their leader, so she often isn't sure which side of Humans VS Oni she should be on, as both sides are not right and she is important to both of them.
- Cloning Blues: Turns out Soko and Keito aren't reincarnations, but clones. Takao is originally also thought to be this. Soko goes through worry about just being a copy that can be "re-done" if she dies, but eventually grows into just being herself and not Soryu.
- Color-Coded for Your Convenience: Soryu is Blue, Suzaku is Red, Byakko is White and Genbu is Black. The colors appear multiple times, in terms of their flames' color or the name of their respective Sanctuary or the Seal placed upon them, needed to gain entry.
- Death of a Child: The Oni do breed and Oni children are seen later on and some die. Soko is horrified when Akira is torturing and willing to kill an Oni child for the sake of gaining information.
- Decapitation Required: The only way to properly kill an Oni is to cut the head off.
- Deliberate Injury Gambit: Soko pulls this on the Black Demon Mask, when they are trying to get to Genbu's Sanctuary. She allows the Demon Mask to impale her on its long horn, but keeps it there steadily and tells it that she will only let him go if it will take them to the inner part of the Sanctuary.
- Earn Your Happy Ending: The last few chapters have the Oni decide to sleep in their Fuji Sanctuary to prevent the volcano from erupting, with Soko going to see Akira one last time before joining them. Hiko ends up blocking her way to the Sanctuary, saying that now that she has gained the power of Soryu herself, they don't need her and Soko is free to remain with Akira. The final page reveals that she and Akira are about to sleep with each other, to rid her of her Soryu powers once and for all and finally become a normal couple.
- Fate Worse than Death: What the true power of the Suzaku does. They can chop an Oni's head off, but instead of killing them, they can cause the head to continue living for eternity.
- The Four Gods: They are families and the chosen heir and current leader are referred to as "the (God)" of the family. Akira is the current Byakko, Soko is Seiryuu (though called Soryu), Takao is Genbu and Keito/Hiko is appointed the new Suzaku.
- Green Aesop: Suddenly seems to get this in the last few chapters. It's revealed that the Oni's great Sanctuary is inside Fuji in Tokyo and its power being used up by Oni sleeping within the Sanctuary keeps it dormant. With more Oni awakening, Fuji is about to erupt. The Oni decide to sleep and keep the humans save for now, because one day the humans will cause nature to go rampant... and then they won't care that Oni are walking around and eating a few humans to feed themselves, since it will count as a way to balance the system.
- Grey-and-Gray Morality: The Humans VS Oni thing. Both sides are presented as merely wanting to live out their lives peacefully, but humans do not tolerate the Oni because they eat humans, where as Oni simply eat humans because it's in their nature. Neither side is shown as perfectly good or perfectly evil, as the Oni are shown eating humans, but the humans are also shown practically committing onicide left, right and center. The conflict is mostly seen by Soko, as she wonders why humans and oni simply can't live side by side.
- Happily Married: It's revealed that this was the case for Soryu and Takao in the past.
- Holy Ground: The Sanctuaries of each of the four families. And they all have their specific properties, like one being a healing spring, another serving as a place of ritual and a main Sanctuary of the family that gives the current leader of the family its true power.
- I Am a Humanitarian: How the Oni tribe feast. The lower Oni eat humans normally, leaving bones and flesh bits behind. The royal Oni, like Soko, are capable of consuming the entire human, including bones, in a much more ethereal and graceful way, that only leaves clothes behind.
- I Just Want to Be Normal: When Soko realizes that she is Soryu's reincarnation, she wishes for nothing more than to be normal.
- Informed Attractiveness: Soryu is repeatedly stated to have been the most beautiful woman in the world, whose mere appearance could stun both men and women. And while Soko is certainly drawn to be beautiful, given this is Shinohara's drawing, she does not look any more beautiful than most other women.
- Luke, You Are My Father:
- In the last few chapters, it's revealed that Keito/Hiko is Soko's/Soryu's daughter, reincarnated from ancient times. And aged up to teen years.
- In a similar vein, it's mentioned almost off-hand that the father is Takao.
- Meaningful Name:
- Soko is written with the kanji for Blue and Child, with her chosen family name also hinting at her Oni heritage of being Soryu. Later, Soko gives Keito the new name of Hiko, written with the Kanji for Crimson and Child, upon naming her the new Suzaku.
- The Black Demon Mask later on joins and is renamed Ou, for the sake of convenience. It can be read as King or as Gold, once again using a color name.
- The location of the Byakko family's home is Kinasa which means Place With No Oni.
- New Transfer Student: The story starts off this way, with Soko attending a new school after moving to the area.
- Ordinary High-School Student: What Soko thinks she is and tries to convince Akira of.
- Plot-Relevant Age-Up: When Keito originally died, Keito was only about 4 years old. Takao decided to clone and bring them to life as a young teenager, about a decade older, because they wanted to see what they'd be like at that age.
- The opposite is done with an Age Down with Soko. When asked why Soko wasn't cloned to around the age of 30 or whatever during their original death, Takao says it's because Soryu was 16 when they got married.
- Reincarnation Romance: Attempted with Takao and Soko, though whether this counts as reincarnation since Takao is a projection that materializes and de-materializes at will and he was never actually reborn is debateable.
- Resurrective Immortality: What Takao seems to have, re-appearing practically unharmed whenever injured or presumed dead. It's actually just part of the Fate Worse than Death listed above.
- The Reveal: Several, throughout the series. Soko is Soryu. Takao is not just a normal human, but actually the Genbu of the northern family. Keito is revealed to be Soryu's sister, though this later proves to be false and Keito is actually Soryu's daughter. Akira and Ryo are half siblings, just like the others of the Byakko family.
- Sacrificial Lion: The Byakko family, and particularly Akira's uncle, proves how evil and serious they are in getting rid of Soryu that they send a bomb to Soko's home, which explodes and kills her parents and brother and destroys her home.
- Star-Crossed Lovers: Soko and Akira see themselves as this, as Soryu was the leader of the Oni tribe and the Byakko family has always been the ones with the duty to kill Oni. Played up a lot in the final few chapters when Soko plans to sleep with the Oni inside Fuji's base to prevent it from erupting, meaning she'll be forever separated from Akira, as the Oni wouldn't awaken until thousands of years later.
- Team Pet: Eventually, the Black Demon Mask from Genbu's Sanctuary tags along with the protagonists, intrigued by Rago's clone Soko. He is mostly helpful in giving information on the lore of the Oni, including vague descriptions where the main Sanctuaries of the other families are, though he doesn't know the specific locations himself.
- Unicorn: The Demon Masks at the Sanctuaries look like unicorns, with a mane of hair and an incredibly long, thin horn. They can shake their mane to cover their body and horn and end up looking like extremely shaggy dogs, though.
- Vacuum Mouth: Doesn't use the mouth but the Byakko's powers include sucking the power and life out of an Oni by touching their heart. The true power of the Byakko upgrades this to being able to send out tiny white vacuum bubbles at Oni and sucking them dry from afar.
- Virgin Power: Not necessarily bound to her for being a virgin, but Soko would lose her status and power as Soryu if she sleeps with a man who is more powerful than she is. This includes Akira, as well as some of the other men of the Byakko family, and is considered one of the ways to turn Soko normal.