Sakutaro Hagiwara is a fictionalized version of the real Japanese poet (1886–1942). He works as an author and regularly visits the Milk Hall newspaper club Kikunyusha. His personality combines intelligence with a quiet demeanor and emotional restraint, often concealing his feelings.

Regarding the murder investigation of sex worker Otaki, Hagiwara develops a specific theory. He proposes Kyosuke Kindaichi developed romantic feelings for Otaki. Hagiwara speculates Kindaichi sought a way for them to be together, but Otaki recognized her freedom was financially impossible given Kindaichi's modest teacher's income. Hagiwara suggests Otaki's love for Kindaichi made intimacy with other clients unbearable, prompting her to pursue a Buddhist monastic life requiring head shaving. He theorizes an accidental, self-inflicted throat injury occurred when a startled cat caused her to jerk a knife held near her head.

Subsequent investigation proves his theory incorrect, confirming Otaki's death as suicide.

Within the story, he belongs to a circle of literary figures aiding detectives. He appears alongside Nomura Kodo and Yoshii Isamu during Kindaichi's arrest in the Otaki case, contributing to the group's discussions on the incident. His narrative involvement occurs primarily within episodic cases rather than driving overarching plot development.