Shirou Ryujoji is a student and the first son of the Ryujoji clan, initially believed to be the second son. His clan harbors a centuries-old vendetta against the Yagyu Jubei lineage, drawing Shirou into their quest for vengeance against Jiyu Nanohana, the current inheritor of the Yagyu Jubei mantle. Their initial encounters occur while wandering a forest, sparking romantic feelings in Shirou despite their opposing allegiances.

Torn between loyalty to his clan and his growing affection for Jiyu, Shirou's conflict peaks when the vengeful spirit Taiko Daiyu, the Ryujoji clan's ancestral leader, possesses his body to confront her. During the possession, Shirou battles internally, ultimately regaining control and forcing Taiko Daiyu into another host. This act of defiance signifies his defection from the clan's cause, prioritizing his connection to Jiyu over familial duty.

One year later, in the second series, Shirou's role substantially diminishes. Aware of his reduced importance amidst conflicts involving the Siberian Yagyu clan and Freesia Yagyu, he attempts to aid Jiyu against new threats but is swiftly defeated, highlighting his inability to match escalating adversaries. His unrequited feelings for Jiyu persist, unresolved as the narrative shifts focus. His background as a Ryujoji heir and possession experience underscore his position, trapped between legacy and personal desire.