**Keiko Mikata**, also identified as Keiko Mekata, is a shrine maiden summoned by the spirit of Taira no Masakado to thwart dark onmyoji Yasunori Kato’s campaign to annihilate Tokyo. Her spiritual protection duties intensify after marrying into the Tatsumiya family, where she adopts the name Keiko Tatsumiya. This union embeds her within the household, positioning her to shield the family from internal supernatural dangers.

Kato fixates on Keiko, envisioning her as a counterpart due to her spiritual power. He provokes her through escalating tactics—unwanted advances and torturous visions—aiming to corrupt her into embracing hatred and becoming his "demonic bride." Kato explicitly declares his intent to transform her into an oni like himself.

When Kato attempts to sacrifice Yukiko Tatsumiya to awaken the Underground Dragon, Keiko pursues him into Tokyo’s subway tunnels. Her intervention aligns with Gakutensoku’s destruction at the dragon’s spiritual core, fracturing Kato’s ritual and freeing Yukiko. Keiko then channels Masakado’s power to banish Kato temporarily.

In their final clash, a wounded Keiko—aided by Feng Shui master Shigemaru Kuroda and Masakado’s spirit—confronts Kato at a desecrated temple outside Tokyo. There, Kato tries to divert the moon’s path to obliterate the city. Rejecting violence, Keiko undergoes a metamorphosis, embodying the bodhisattva Kannon (Kwannon). Her manifestation engulfs Kato in boundless compassion, dissolving the ancient curse that fueled his hatred and culminating in his dissipation.