Sae Shingyouji is Shinji's childhood friend, girlfriend, and future promised wife. Their mutual vow of marriage includes Sae's commitment to abstain from sexual activity until after their wedding. This promise fundamentally shapes her character and significantly influences Shinji's choices throughout the story.

Their relationship faces immense strain when Shinji must locate a woman bearing a specific birthmark, one that manifests only after sexual climax, to avoid inheriting his grandfather's empire. Sae's vow creates a profound conflict for Shinji, forcing him to reconcile his commitment to her with the necessity of pursuing intimate encounters during his search.

Sae serves as Shinji's primary emotional anchor and embodies his envisioned future. Her presence fuels his determination to resolve the inheritance dilemma without breaking their bond. However, her direct participation in the central conflict remains limited, as Shinji's quest requires interactions with numerous other women while he strives to remain faithful to Sae.