The youngest of identical twin sisters, Yayoi Matsunaga was horribly disfigured in a fire when she was five years old, which also killed her parents. Her older twin, Katsumi, was spared because of a nightbreed who wanted her unharmed body. As a result, Katsumi was forced by the nightbreed to kill innocent people and rip off their faces. Yayoi, not knowing of what was going on, grew up with her sister, all the while hating Katsumi for being unharmed.    All this changed when Tatsuhiko Shido and Guni visited the inn that the sisters ran together. At first, Shido thought that it was Yayoi that was possessed by the nightbreed, mainly because of the hostile way that Yayoi treated Katsumi. But later, Yayoi had accessed a secret room in the basement of the inn. It was there that she, along with Shido, had found the faces in which the nightbreed that possessed Katsumi ripped off. Both were horrified, and Katsumi was so anguished to find that her sister had discovered her shameful secret, that she dropped the lantern she was holding, starting a fire. There was a brief battle between the nightbreed and Shido, but Katsumi decided to end it all by throwing herself into the flames. Yayoi, at last acknowledging her love for her sister, tried to save her, but Katsumi, knowing that the nightbreed would only accept a beautiful body, leapt into the fire, causing the nightbreed to leave her body, which Shido destroyed soon after. Both sisters were saved from the burning building, but Katsumi soon died from her injuries. Before that, however, Katsumi used the last of the nightbreed's power to give Yayoi her soul... as well as her face.    Saddened by the loss of her sister, Yayoi soon joined the N.O.S., an agency whose duty is to investigate and eliminate nightbreed threats. She retains Shido to aid her in her quest, although her flirtatious attitude often causes Riho to get very jealous at her. She uses a standard 9mm automatic pistol loaded with silver-jacketed bullets, and fitted with an entry light.    Because of the presence of Katsumi's soul within her, Yayoi has gained immunity from a vampire's bite. Because of this, she is able to give Shido, and later Riho, the blood they need to survive.     (Source: Absolute Anime)

Shido Tatsuhiko is not only a private eye... he is also a vampire with no real memory of his past. Joined by Yayoi Matsunaga, a female government agent, Riho Yamazaki, an orphaned teenage girl working as his girl Friday and Guni, a little green imp, Shido must face demonic creatures known as Nightbreed. Meanwhile, Cain, the vampire who made him what he is now, is seeking him...
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(Source: Anime News Network)

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She uses the name So-Yeon as her stage name.