Yōichirō Tatsumiya is the brother of Yukari Tatsumiya, a young woman targeted for her spiritual lineage connected to the vengeful spirit Taira no Masakado. After Yukari's traumatic kidnapping and return by Yasunori Kato, Yōichirō expresses conflicted emotions toward her through an act of non-consensual incest, resulting in Yukari's pregnancy. This incident diverges from the original novel's consensual depiction of their relationship.

Yōichirō becomes the father of Yukiko Tatsumiya, though Kato initially believes himself her biological parent due to his earlier magical interference. Following a ten-year time skip, Yōichirō resides in the Tatsumiya household when Kato attempts to abduct Yukiko for a ritual to awaken Masakado's spirit.

He marries Keiko Mekata, a shrine maiden summoned by Masakado to oppose Kato, after encountering her during prayers. This marriage integrates Keiko into the Tatsumiya family, enabling her ongoing spiritual protection against Kato.

In the anime adaptation, Yōichirō exhibits apathy toward the conflict, driven by fear of supernatural forces and emotional stress. His most extreme action occurs when he attempts to strangle Yukiko upon her discovery of her parentage, an act interrupted by Jun'ichi Narutaki. This contrasts sharply with his portrayal in the live-action film *Tokyo: The Last Megalopolis*, where he actively defends Tokyo and Yukari, erects protective statues at Nihonbashi Bridge, and valiantly offers his blood at Masakado's grave site to appease the spirit, with no depiction of sexual assault.