Tetsuo Ishii, Ayase Yukiya's 20-year-old maternal cousin and a student, racks up heavy gambling debts. Unable to repay, he betrays his cousin, orchestrating Ayase's sale at a sex slave auction specifically to settle his own financial obligations. This calculated act prioritizes self-preservation over family loyalty.

Ayase is sold for 120 million yen to loan shark Somuku Kanou. Kanou's intervention resolves Tetsuo's debts, ending his immediate crisis. Tetsuo's mother, Rikako Ishii (Ayase's aunt), is identified, though her involvement or awareness remains unspecified. His character is defined by this betrayal, with no documented long-term consequences, moral reckoning, redemption, or further narrative evolution across the media.