"Be my ideal girl..." It's been two months since childhood friends, Yuuka and Kouta, started dating. However, due to their inexperience, their "first time" together doesn't go as planned. Enter Kouta's younger sister, determined to ensure the couple's happiness... No matter the cost... A boy, a girl, and his younger sister... ___ **Note:** This comic has the GL tag because it contains yuri. The mangaka has also stated that it "isn't a yuri manga". There is also heterosexual sex in this, so this serves to inform readers that this isn't a purely yuri manga.