My girlfriend's older sister is a hikikomori. Even when we occasionally meet at her house, we only exchange greetings, but she always stares at me with those gloomy eyes — the epitome of an introverted gloomy girl.

One day, I accidentally witnessed her masturbating... Her usually plain-clothed body with massive breasts, slim waist, and huge ass made me instantly rock hard!

From that day on, without my girlfriend knowing, I ended up in a mutual masturbation relationship with her older sister!