Keiko Mekata serves as a young shrine maiden near the Ministry of Finance during Tokyo's 1927 reconstruction. Possessing significant spiritual power as a miko, she was specifically summoned by the spirit Taira no Masakado to thwart the sorcerer Yasunori Kato's destructive plans for the city. While praying at Masakado's shrine, she meets and eventually marries Yoichiro Tatsumiya. Upon entering the Tatsumiya household as Keiko Tatsumiya, she provides ongoing spiritual protection for the family from within its walls.

As a member of the Tatsumiya family, Keiko actively defends them against Kato's supernatural assaults. When Kato kidnaps Yukiko Tatsumiya to sacrifice her for awakening the Underground Dragon, Keiko confronts him directly at the household before pursuing him underground. Her actions during this clash contribute to disrupting Kato's ritual, especially when the robot Gakutensoku destabilizes the dragon's energy veins. Keiko subsequently channels Masakado's power to banish Kato temporarily.

After Kato retreats to a desecrated temple outside Tokyo, he attempts a final assault by manipulating the moon's path to awaken the Firmament Dragon. Keiko, aided by feng shui expert Shigemaru Kuroda and Masakado's spirit, confronts him once more. Rather than physical combat, she channels the power of the bodhisattva Kannon. This act of boundless compassion neutralizes the curse and hatred sustaining Kato, causing him to dissipate completely.

Kato develops an obsessive fixation on Keiko, repeatedly targeting her with disturbing visions, physical advances—such as employing his shikigami to grope her—and psychological torment intended to provoke intense hatred. His objective is to corrupt her into becoming an oni like himself, envisioning her as his demonic bride in Hell, establishing Keiko as a persistent target of his supernatural and psychological aggression.