Keiko Mekata, who becomes Keiko Tatsumiya upon marriage, is a central figure in the narrative of Doomed Megalopolis. Her background is that of a shrine maiden, or miko, with significant spiritual power, residing near the Ministry of Finance during the reconstruction of Tokyo following the Great Kanto Earthquake of 1923. She is chosen by the vengeful spirit of the samurai Taira no Masakado to be his protector and to act against the sorcerer Yasunori Kato, who seeks to destroy the city by awakening Masakado's power against his will. Her mission leads her to the Tatsumiya household, where she meets and marries Yoichiro Tatsumiya, becoming a member of the family specifically to provide ongoing spiritual protection for them against Kato's schemes.

Keiko's personality is defined by a deep sense of duty, spiritual strength, and formidable resolve in the face of a terrifying adversary. She is not an aggressive warrior but a figure of focused spiritual authority. When confronted by Kato, who subjects her to disturbing visions, psychological torment, and unwanted physical advances in an attempt to corrupt her into becoming his demonic bride, she resists the pull of hatred and despair. Her motivations are rooted in her role as a protector. She is driven to shield the vulnerable members of the Tatsumiya family, particularly Yukari and her psychic daughter Yukiko, from Kato's predation. Her purpose is to foil Kato's elaborate rituals aimed at causing catastrophic events, such as awakening the Underground Dragon to level Tokyo or manipulating the moon's orbit to summon the Firmament Dragon.

Within the story, Keiko's role evolves from a divinely appointed agent to the family's primary defender and, ultimately, to a figure of transcendent compassion. Her key relationships are central to this evolution. She marries Yoichiro Tatsumiya, an act that formally integrates her into the family and its fate. Her primary antagonist is Yasunori Kato, who becomes fixated on her as a worthy counterpart, attempting to force her to surrender to her own darkness. She is also aided by Shigemaru Kuroda, a feng shui expert, with whom she collaborates to counter Kato's mystical attacks. The spirit of Taira no Masakado serves as her supernatural patron, granting her power and guidance in her fight against Kato.

Keiko undergoes significant development from a powerful shrine maiden to a figure of almost divine mercy. In her early confrontations, she engages in direct magical combat, using shikigami and channeling Masakado's power to banish Kato and protect Yukiko from being sacrificed to the Underground Dragon. In one fierce battle, she manages to injure Kato by throwing a knife that pierces his hand and gouges out one of his eyes. However, her final confrontation represents a profound shift away from violence. As Kato attempts to cause a celestial catastrophe, Keiko channels the power of the bodhisattva Kannon, the goddess of mercy, rather than a wrathful spirit. By embodying boundless compassion, she is able to neutralize the ancient curse of hatred that fuels Kato's immortality. She embraces him, and their opposing natures—her compassion and his hatred—annihilate each other, causing them both to disappear in an explosion of light and finally breaking his hold on Tokyo. As a shrine maiden, her most notable abilities include summoning shikigami (spirit familiars) for battle and defense, wielding spiritual energy to purify evil, and channeling the power of powerful spirits like Masakado and the bodhisattva Kannon.