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A lovely young exorcist who soon turns into the very thing she had spent her life fighting against.
Yomi Isayama was a young girl who worked for a team of exorcists out of Japan's Ministry of the Environment. In Ga-Rei Zero, it's revealed that Yomi carried a sword called the "shishi-oh" crafted by the swordsmith Michael and used to to slaughter evil spirits or to summon her own powerful spirit beast named Rangurren.
Yomi was adopted into the Isayama family, but stood to inherit the family name and the status among the country's exorcists once her father died. She also became an unofficial sister to Kagura Tsuchimiya when she came to live in her house. The two of them got along pretty well together, and she was even arranged to be married to Noryuki Izuna, another exorcist.
However, tragedy slowly consumed her life, starting when the freelance exorcist Mei grew jealous of Yomi's status and killed her father in order to take the Isayama inheritance for her father and her own bloodline. Mei even took her Shishi-oh from her, saying she was not worthy of it. In a later mission, Yomi ran into Mei as she was posessed by the "Death Stone," or sesshouseki. During the battle, Mei admits to killing Yomi's father, and even laughs at the memory, but then she begs for mercy when Yomi has her pinned at the end of the fight. Yomi tells her to shut up, kills her, and takes back her sword.
Just as Yomi returns from the fight, she runs into a mysterious white-haired boy who tells her more about the Death Stone, and then proceeds to cut up most of her nerves with several thrown blades. Yomi ends up crippled in a hospital bed, unable to speak and barely able to move more than her fingers. The other exorcists, including Mei's father, demand to know if she murdered Mei, but she can't answer. Noriyuki's father also comes in to inform her that he's severing the marriage between her and Nori, while Noriyuki is busy searching for answers about the one who attacked her. Kagura visits and tries her best to cheer Yomi up. But one night, the white-haired boy reappears beside her bed, and uses the power of the Death Stone to heal her back to full strength.
Later, Yomi takes out her frustration on Mei's father for his daughter's sins, and then continues by slaughtering all the exorcists she can find with her sword and Rangurren. During one attack in Tokyo, she slowly tortures one of the exorcists to death in front of Noriyuki, and begs him to kill her, but Nori can't bring himself to do it. A few minutes later, she puts Kagura in the same position, but Kagura can't muster the courage to end her life either, giving Yomi the chance to slice through her father's chest. After the battle, Yomi has an attack of conscience and tries to take her own life, but the Death Stone won't let her, forcing her to keep fighting the exorcists until she can find one with the strength to kill her.
After slaughtering several more people, the surviving members of the exorcist team finally track down Yomi, and Kagura finally decides to fight her for real. Yomi confesses her love for Kagura before she finally dies from Kagura's attack.
Yomi reappears in the Ga Rei manga as a vengeful spirit with a Death Stone still stuck inside her, antagonizing Kagura, Nori, and the newly-minted exorcist Kensuke Nimoura. She hatches a plot to release the thing sealing vengeful spirits in Tokyo, and constantly battles the exorcists. However, during a pentultimate battle, Kensuke manages to wound her with his sword, and the white-haired boy takes her Death Stone back, finally killing her for good.
Sometime after that, a girl named Izumi shows up in Ken and Kagura's classroom. She looks incredibly similar to Yomi, but neither she nor Kagura seem to remember any of their relationship until a battle with evil spirits brings Yomi's personality to the surface in Izumi's body.