Yoichiro Tatsumiya is Yukari Tatsumiya's brother and wields significant influence at Tokyo's Ministry of Finance. His relationship with Yukari involves profound emotional conflict, escalating to a non-consensual incestuous encounter while she was vulnerable following an assault by antagonist Yasunori Kato. This act led to Yukiko Tatsumiya's conception, though Yoichiro initially believed Kato was the father.

He later marries Keiko Mekata, a shrine maiden summoned by the spirit of Taira no Masakado to defend Tokyo. This marriage integrates Keiko into the Tatsumiya household, where she offers spiritual protection against supernatural threats. Yoichiro's relationship with his daughter Yukiko becomes strained; when she uncovers his paternity and the circumstances of her conception, he attempts to strangle her in a crisis, halted by Junichi Narutaki's intervention.

The original novels depict Yoichiro's relationship with Yukari as consensual, contrasting with the anime adaptation's non-consensual portrayal. Professionally, he mediates between industrial and spiritual conflicts within Tokyo's governance. His character evolves from initial apathy toward spiritual warfare—rooted in fear of incomprehensible forces—toward more active, though inconsistent, participation in defending Tokyo during later narrative arcs.