Seiya Shinjuku is a handsome male club host with sand-colored or grey hair swept to the right and brown or grey eyes. He was initially involved in a relationship with Julia Katsushika. This relationship ended when Ayaka Toshima, harboring an obsessive infatuation with him, tricked him into wearing an SCM device. Enslaved to Ayaka, he was forced to break off with Julia and expel her from their shared apartment.

Following his enslavement, Seiya resided under Ayaka's control until Yuuga Ohta challenged her to an SCM game. Seiya deliberately lost this game, transferring his enslavement to Yuuga to escape Ayaka. Subsequently encountering Julia again, Seiya proposed reconciliation and cohabitation. Julia rejected him, revealing her enslavement to Ryuuou Edogawa.

Later, Julia exploited the "unaware opponent" rule to initiate an SCM game with Seiya. He lost upon releasing her hand, resulting in his temporary enslavement to Julia before she transferred ownership to Ryuuou. Seiya then joined Ryuuou's group of slaves, aligning with figures like Ataru Chuuou and Zenichi Bunkyou.

During this period under Ryuuou's control, Julia disclosed she was pregnant with Seiya's child. Available information does not confirm whether Seiya learned of this pregnancy.