Seishin Gyōsha (Kaze no Stigma fanfiction)

There is more than meets the eye to a teenage girl from a rich, well-off family in Japan.
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When Yasuko Himura is called to meet an old friend of hers, Juugo Kannagi, she will find that he needs a rather huge favor from her. He needs her to change herself in order to protect his family. She agrees and drinks a potion only she has the recipe for in order to make herself into Juugo's biological daughter.

Yasuko Himura, the Spirit Contractor and daughter of the Spirit King of Spirits, is reborn as Umeko Kannagi, eldest daughter of Juugo, older sister to Ayano, and distant cousin to Ren and Kazuma.

When Kazuma returns, Umeko is thrown into a new world of true love, danger, and power. And everything will change as her secret is unearthed.