Spoiler, click to view
Kogue Yuuya a teacher in middle school section (probably he is home teacher of 1-B class). He was in love with Rie from fives years. They first met in this same school when he was in the 3 grade of high school and she was in the 1 grade of middle school. That was also time when Kogue, for first time confessed his love to Rie but was rejected.
