Rika and I have been dating since high school. Perhaps because we've been in love for so long, our feelings for each other are no longer the same as before. Recently, Rika has gotten to know a fifth-year man who surpasses me in every aspect, making Rika waver. That man is preparing to travel and invited Rika to go along; she agreed and took this opportunity to confirm our current status. Although Rika didn't say anything, I could still sense that something was happening. While sitting alone in a park near my house, Sister Konan walked by. She knew everything, sat down to listen to my concerns, chatted, and helped improve my mood. On the day Rika was traveling, she waited for me outside the door and offered to take Rika's place as my girlfriend for three days! And during those three days, I felt like I returned to the early days of falling in love. I got to know more about her and gradually developed feelings for her. I tried to confess my feelings, but she rejected me, saying I was Rika's boyfriend, her sister, and that nothing could ever happen between us...