The premise is simple (and disturbingly commonplace in modern manga): our heroine is a nasty immortal being and can't stand it. In this particular story, our heroine is Kiryuu Souko, a young woman who wakes up in a normal, modern Japanese household one morning and lives through a few days of normal life ... only to discover soon afterward that she wasn't the natural daughter of the family, and in fact had never met her "parents" until that fateful morning. In fact, she discovers quickly, she is the dreaded recurrence of the goblin/demon leader Ragou (the "Sou-ryuu" of the East), a fiendish single-horned woman of exceeding beauty who once ruled the mighty hordes of demons that roamed the earth. She is immortal (she learns), powerful, and in near-perpetual need of human life energy for sustenance.
Souko finds most of this out from Akira, who is the Byakko "White Tiger" of the (super wealthy) West House that has survived the years since the demon-ages; he is the one whom destiny has appointed to hunt down the dreaded Sou-ryuu. Unlike the demon Houses, his line is mortal and limited to human lifespans; his family is mostly "human," with the exception of having some super-human abilities as well as the power to suck the life out of demons in the same way demons suck the life out of humans. His House is at eternal war with the demonkind, and especially the Sou-ryuu (aka Souko).
Unfortunately for Souko and Akira, they fall in love. The Sou-ryuu, according to legend, is so beautiful she steals the souls of all who look upon her. (Did we mention this was a girls' comic?)
Souko is not at all happy about being a resurrected Sou-ryuu. She spends a great deal of time denying that it is possible, and when she finally accepts it, she spends all of her time worrying about it and trying to figure out a way to stop being the Sou-ryuu. If she can't find a way, she reasons, she'll just let herself be killed by Akira. But ideally, she'd love to become human and live a normal life as Akira's wife (yes, that old familiar shoujo manga theme).
Akira, for his part, is at first unconvinced of her sincerity, but finds himself believing her more and more. He is, however, thwarted in his attempts to help her by his meddling family, who are either inclined to try to take his "Byakko" title, or who are so steeped in tradition that they cannot conceive of trying to help a would-be human Sou-ryuu.
Souko has her own family problems. Despite the occasionally glacial pace at which she figures things out (she's not terribly bright at times), we soon learn that Souko is not the only goblin/demon alive in modern Japan. Among others are the darkly handsome and brooding Takao, the powerful leader of the North House (following traditional Chinese mythology for the Four Directions), and the one who (it turns out) cloned Souko from a strand of hair from the head of the original Souryuu, and Hiko, who is apparently a clone of the Souryuu's sister. Both (Hiko less willingly) are intent on having Souko fulfill her potential as the new leader of the demon clans. Souko's powers (particularly the one of converting humans into loyal demons) would enable demonkind to take over the earth.
The story then follows Souko and Akira as they investigate the ancient records and ruins of the old demon clans, trying to find a way to make Souko human (they find a way, but they can't implement it yet), a way to stop the re-cloning of new Sou-ryuus, and a way to stop Takao's hordes of deadly demon-people.
As much as Souko gets to angst about her life, Akira also gets to brood in private over Takao's hidden love for Souko --- which Souko herself doesn't see. Souko at one point accidentally falls into Takao's hands, and she is brainwashed into helping him with his work. Partially as a result, she gains a slow but growing sympathy for "her people" --- after all, in these books, demon/goblin-kind wind up acting a lot like humans (with the excetion they depend on humans for sustenance, and that they seem to know their bosses instinctively). (From a completely outside perspective, it would appear that converting humans into demons simply replaces the body's original soul with a very similar soul --- except one inclined to dining on people, and one imbued with greater powers.)
Though at times highly implausible, and at times deeply frustrating because of the heroine's continuing inability to reach mental conclusions, the story nevertheless remains engrossing --- not necessarily an easy feat. The author uses strands of friendship, romance, horror, moral paradoxes, ethical dilemnas, and plenty of good old action and suspense to keep the story moving. Moreover, it is clear that the author had a definite story and a particular end in mind for Souko. Unlike some manga that wander belatedly to an emergency conclusion, the secrets that come out at the very end make it clear that they had been lying hidden throughout most of the story. And yes, there are little secrets that dribble out here and there, keeping the interest up.
Overall, despite some of the "fluff," Ao no Fuuin pulls off a minor feat in entertainment.