| Character | Role | Voice Actor (Seiyuu) | Notes | | :--- | :--- | :--- | :--- | | | The main heroine; the protagonist's cool, expressionless but voluptuous sister-in-law. | Riho Sugiyama | Visits her brother-in-law while her sister is away, leading to a series of intense sexual encounters. | | Onii-san (お兄さん) | The male protagonist; Nanami's older brother-in-law (the husband of Nanami's older sister). | Aoki Hikaru | Initially hesitant, he quickly succumbs to Nanami's advances and becomes the central participant in the story. |
The story centers on a young man and his girlfriend’s younger sister. The sister, who has the exact same body type as her older sister (i.e., “和姐姐同样身材的妹妹”), comes to the protagonist to “repay him for always taking care of her.” What follows is a sexual encounter where the protagonist ignores the sister’s repeated request to use a condom – hence the title “I told you to put on a condom, didn’t I?” | Character | Role | Voice Actor (Seiyuu)