Sodachi Riddle, Part Two

A day of the summer, 5 years ago. Koyomi visited a vacant house with a invitation letter. A girl was waiting for him, and then, they studied mathematics together. He kept going to the house everyday, but a last day of summer vacation, she suddenly left without saying anything to him. --- Listening to the story, Ougi reminds Koyomi that she and Sodachi are...