Iori Yagami

1800 years ago, there were three predominant clans in Japan, which were the Kusanagi clan, the Yasakani clan, and the Yata clan.

Each of those clans was the guardian of one of the sacred treasures.

The Kusanagi clan guarded the Kusanagi sword, the Yasakani clan guarded the Yasakani jewel, and the Yata clan protected the Yata mirror.

With Orochi's attack against these three clans, they managed to seal Orochi, who was an evil entity.

However, Orochi had his wife, named Ame, who had been manipulated and blinded by Orochi's power. She attacked Yasakani's wife and made her believe it was Kusanagi.

Blinded by hatred, Yasakani allied himself with Orochi, obtaining the purple flame. After this event, Yasakani changed his surname, and his successors would bear the name Yagami.

Yasakani clashed with Kusanagi, becoming his lifelong enemy, and later Kusanagi defeated Ame in battle.

What she didn't know was that she had a portion of Orochi's power sealed inside her, which would be transferred to her direct reincarnation.

Now, in the present day, Kyo Kusanagi was the reincarnation of Kusanagi, Iori Yagami that of Yasakani, and Chizuru Kagura that of Yata.
Chizuru and Kusanagi had found Ame's reincarnation, and she had been adopted as Kyo's sister, to train her on the side of good.
Kyo possessed fire powers, while she had telekinesis and mind control powers.

Kyo and Iori were enemies, so obviously the girl was also forced to be against him.

Enemies?