During his time in middle school with Futaba, Kou Tanaka was rather small and gentle. He became very fond of her throughout their experiences together but seemingly disappeared during a summer break. Making an appearance in Futaba's high school, it is revealed that he was unable to attend the summer festival because of familial issues. No longer "Kou Tanaka", he is now referred to as "Kou Mabuchi". His personality has also taken a turn, for he is now more masculine, sarcastic, and crude. In addition, he has extremely pessimistic views about himself and about his brother. Despite these changes, it is evident that Kō is still an overall kind person. When Kominato visited his house and found his secret box full of things from middle school, Kou confessed to Kominato that he still likes Futaba. To him, Futaba seems to be a remainder of his happier days—the days before his family experiences turmoil.
Source from Wikipeida