Seiji belongs to the Oozora Group, a yakuza organization under Ibari Oozora's leadership. His frightening facial features sharply contrast his timid, faint-hearted demeanor. Though intimidating in appearance, he exhibits a gentle, easily startled nature. A recurring task involves waking Kousaku Sakamoto each morning by leveraging his alarming visage to jolt him awake.

Seiji navigates a romantic subplot upon receiving his first love letter from Hanako, a friend of Kaori Hanazono. Hanako developed feelings after observing him at the beach. This sparks significant concern in Seiji, who fears his yakuza affiliation renders him unfit for marriage. The relationship underscores his deep-seated insecurity about his profession's impact on personal life.