Ryujoji Shiro features prominently across both seasons of the Jubei-chan narrative. He is a skilled and elegant kendo club champion at Hontsuru Junior High School. His initial encounter with a lost Jiyu Nanohana in a forest sparks his immediate romantic interest, establishing him alongside Bantaro Sanbonmatsu as one of Jiyu's primary suitors.

Shiro belongs to the Ryujoji clan, which harbors a centuries-old grudge against Yagyu Jubei. Raised within this environment of vengeance, he operates under the misconception of being the family's second son. The clan actively seeks to defeat the reincarnation of Yagyu Jubei, with Shiro's older brother, Hajime Ryujoji, positioned as the heir apparent to lead their forces against Jiyu.

During the first season, Shiro rejects his clan's vendetta, consciously distancing himself from their mission to instead support and protect Jiyu. This protective role continues into the second season, where he demonstrates increased agency by actively collaborating with Bantaro to locate a missing Jiyu and confronting threats against her, including the Siberian Yagyu clan and Freesia Yagyu.

A significant revelation confirms Shiro is actually the Ryujoji family's firstborn son, contrary to his long-held belief. This discovery does not alter his commitment to supporting Jiyu or his opposition to the clan's vengeance. His narrative consistently involves his romantic pursuit of Jiyu, his internal conflict over family legacy, and his ultimate choice to prioritize personal loyalty over inherited conflict.