Forged as a tanto by swordsmith Awataguchi Yoshimitsu, Yagen Tōshirō derives his name from a historical demonstration where he cleanly sliced through a stone medicinal yagen yet remained intentionally dulled at the edge to safeguard his wielder from accidental injury—a design prioritizing both efficacy and user protection.

He belongs to the Awataguchi school alongside brothers Namazuo Tōshirō and Ichigo Hitofuri, sharing an especially close bond with the latter, whom he affectionately calls "Ichi-nii." Battlefield experience as a weapon forged his calm and collected demeanor, positioning him as a stabilizing anchor in group dynamics. These same battlefields granted him extensive knowledge of medicinal practices and first aid.

Among swords tied to Oda Nobunaga in the Odagumi group, Yagen uniquely carries no unresolved trauma from their former master. This emotional resilience allows him to serve as a grounding figure, offering targeted support to Souza Samonji during struggles with memories of Nobunaga's death at Honnōji. He consistently operates as the ensemble's voice of reason.

Assigned to the Third Team, he participates in missions against Historical Revisionists. While specific combat abilities remain unelaborated, his observational precision and pragmatic outlook—honed through historical battlefield exposure—enhance the team's operational cohesion.

His visual design remains consistent across adaptations. In the Honnōji-centric anime narrative, his role remains secondary compared to swords directly linked to Nobunaga's legacy. He appears within the citadel's community and features among the silhouetted secondary characters in the ending sequence.